Write a blog post for Color Spaces (sass#671)

See sass/sass#2831

Co-authored-by: Miriam Suzanne <miriam@oddbird.net>

source/blog/034-request-for-comments-color-spaces.md

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+---
+title: "Request for Comments: Color Spaces"
+author: Miriam Suzanne and Natalie Weizenbaum
+date: 2022-09-21 13:00 PST
+---
+
+There's been a lot of exciting work in the CSS color specifications lately, and
+as it begins to land in browsers we've been preparing to add support for it in
+Sass as well. The first and largest part of that is adding support for *color
+spaces* to Sass, which represents a huge (but largely backwards-compatible)
+rethinking of the way colors work.
+
+Historically, all colors in CSS have existed in the same color space, known as
+"sRGB". Whether you represent them as a hex code, an `hsl()` function, or a
+color name, they represented the same set of visible colors you could tell a
+screen to display. While this is conceptually simple, there are some major
+downsides:
+
+* As monitors have improved over time, they've become capable of displaying more
+  colors than can be represented in the sRGB color space.
+
+* sRGB, even when you're using it via `hsl()`, doesn't correspond very well with
+  how humans perceive colors. Cyan looks noticeably lighter than purple with the
+  same saturation and lightness values.
+
+* There's no way to represent domain- or device-specific color spaces, such as
+  the [CMYK] color space that's used by printers.
+
+  [CMYK]: https://en.wikipedia.org/wiki/CMYK_color_model
+
+Color spaces solve all of these problems. Now not every color has a red, green,
+and blue channel (which can be interpreted as hue, saturation, and lightness).
+Instead, a every color has a specific *color space* which specifies which
+channels it has. For example, the color `oklch(80% 50% 90deg)` has `oklch` as
+its color space, `80%` lightness, `50%` chroma, and `90deg` hue.
+
+## Color Spaces in Sass
+
+Today we're announcing [a proposal for how to handle color spaces in Sass]. In
+addition to expanding Sass's color values to support color spaces, this proposal
+defines Sassified versions of all the color functions in [CSS Color Level
+4][color-4].
+
+[a proposal for how to handle color spaces in Sass]: https://github.com/sass/sass/blob/main/proposal/color-4-new-spaces.md
+
+### Rules of Thumb
+
+There are several rules of thumb for working with color spaces in Sass:
+
+* The `rgb`, `hsl`, and `hwb` spaces are considered "legacy spaces", and will
+  often get special handling for the sake of backwards compatibility. Colors
+  defined using hex notation or CSS color names are considered part of the `rgb`
+  color space. Legacy colors are emitted in the most compatible format. This
+  matches CSS's own backwards-compatibility behavior.
+
+* Otherwise, any color defined in a given space will remain in that space, and
+  be emitted in that space.
+
+* Authors can explicitly convert a color's space by using `color.to-space()`.
+  This can be useful to enforce non-legacy behavior, by converting into a
+  non-legacy space, or to ensure the color output is compatible with older
+  browsers by converting colors into a legacy space before emitting.
+
+* The `srgb` color space is equivalent to `rgb`, except that one is a legacy
+  space, and the other is not. They also use different coordinate systems, with
+  `rgb()` accepting a range from 0-255, and `srgb` using a range of 0-1.
+
+* Color functions that allow specifying a color space for manipulation will
+  always use the source color space by default. When an explicit space is
+  provided for manipulation, the resulting color will still be returned in the
+  same space as the origin color. For `color.mix()`, the first color parameter
+  is considered the origin color.
+
+* All legacy and RGB-style spaces represent bounded gamuts of color. Since
+  mapping colors into gamut is a lossy process, it should generally be left to
+  browsers, which can map colors as-needed, based on the capabilities of a
+  display. For that reason, out-of-gamut channel values are maintained by Sass
+  whenever possible, even when converting into gamut-bounded color spaces. The
+  only exception is that `hsl` and `hwb` color spaces are not able to express
+  out-of-gamut color, so converting colors into those spaces will gamut-map the
+  colors as well. Authors can also perform explicit gamut mapping with the
+  `color.to-gamut()` function.
+
+* Legacy browsers require colors in the `srgb` gamut. However, most modern
+  displays support the wider `display-p3` gamut.
+
+### Standard CSS Color Functions
+
+#### `oklab()` and `oklch()`
+
+The `oklab()` (cubic) and `oklch()` (cylindrical) functions provide access to an
+unbounded gamut of colors in a perceptually uniform space. Authors can use these
+functions to define reliably uniform colors. For example, the following colors
+are perceptually similar in lightness and saturation:
+
+```scss
+$pink: oklch(64% 0.196 353); // hsl(329.8 70.29% 58.75%)
+$blue: oklch(64% 0.196 253); // hsl(207.4 99.22% 50.69%)
+```
+
+The `oklch()` format uses consistent "lightness" and "chroma" values, while the
+`hsl()` format shows dramatic changes in both "lightness" and "saturation". As
+such, `oklch` is often the best space for consistent transforms.
+
+#### `lab()` and `lch()`
+
+The `lab()` and `lch()` functions provide access to an unbounded gamut of colors
+in a space that's less perpetually-uniform but more widely-adopted than OKLab
+and OKLCH.
+
+#### `hwb()`
+
+Sass now supports a top-level `hwb()` function that uses the same syntax as
+CSS's built-in `hwb()` syntax.
+
+#### `color()`
+
+The new `color()` function provides access to a number of specialty spaces. Most
+notably, `display-p3` is a common space for wide-gamut monitors, making it
+likely one of the more popular options for authors who simply want access to a
+wider range of colors. For example, P3 greens are significantly 'brighter' and
+more saturated than the greens available in sRGB:
+
+```scss
+$fallback-green: rgb(0% 100% 0%);
+$brighter-green: color(display-p3 0 1 0);
+```
+
+Sass will natively support all predefined color spaces declared in the Colors
+Level 4 specification. It will also support unknown color spaces, although these
+can't be converted to and from any other color space.
+
+### New Sass Color Functions
+
+#### `color.channel()`
+
+This function returns the value of a single channel in a color. By default, it
+only supports channels that are available in the color's own space, but you can
+pass the `$space` parameter to return the value of the channel after converting
+to the given space.
+
+```scss
+$brand: hsl(0 100% 25.1%);
+
+// result: 25.1%
+$hsl-lightness: color.channel($brand, "lightness");
+
+// result: 37.67%
+$oklch-lightness: color.channel($brand, "lightness", $space: oklch);
+```
+
+#### `color.space()`
+
+This function returns the name of the color's space.
+
+```scss
+// result: hsl
+$hsl-space: color.space(hsl(0 100% 25.1%));
+
+// result: oklch
+$oklch-space: color.space(oklch(37.7% 38.75% 29.23deg));
+```
+
+#### `color.is-in-gamut()`, `color.is-legacy()`
+
+These functions return various facts about the color. `color.is-in-gamut()`
+returns whether the color is in-gamut for its color space (as opposed to having
+one or more of its channels out of bounds, like `rgb(300 0 0)`).
+`color.is-legacy()` returns whether the color is a legacy color in the `rgb`,
+`hsl`, or `hwb` color space.
+
+#### `color.is-powerless()`
+
+This function returns whether a given channel is "powerless" in the given color.
+This is a special state that's defined for individual color spaces, which
+indicates that a channel's value won't affect how a color is displayed.
+
+```scss
+$grey: hsl(0 0% 60%);
+
+// result: true, because saturation is 0
+$hue-powerless: color.is-powerless($grey, "hue");
+
+// result: false
+$hue-powerless: color.is-powerless($grey, "lightness");
+```
+
+#### `color.same()`
+
+This function returns whether two colors will be displayed the same way, even if
+this requires converting between spaces. This is unlike the `==` operator, which
+always considers colors in different non-legacy spaces to be inequal.
+
+```scss
+$orange-rgb: #ff5f00;
+$orange-oklch: oklch(68.72% 20.966858279% 41.4189852913deg);
+
+// result: false
+$equal: $orange-rgb == $orange-oklch;
+
+// result: true
+$same: color.same($orange-rb, $orange-oklch);
+```
+
+### Existing Sass Color Functions
+
+#### `color.scale()`, `color.adjust()`, and `color.change()`
+
+By default, all Sass color transformations are handled and returned in the color
+space of the original color parameter. However, all relevant functions now allow
+specifying an explicit color space for transformations. For example, lightness &
+darkness adjustments are most reliable in `oklch`:
+
+```scss
+$brand: hsl(0 100% 25.1%);
+
+// result: hsl(0 100% 43.8%)
+$hsl-lightness: color.scale($brand, $lightness: 25%);
+
+// result: hsl(5.76 56% 45.4%)
+$oklch-lightness: color.scale($brand, $lightness: 25%, $space: oklch);
+```
+
+Note that the returned color is still emitted in the original color space, even
+when the adjustment is performed in a different space.
+
+#### `color.mix()`
+
+The `color.mix()` function will retain its existing behavior for legacy color
+spaces, but for new color spaces it will match CSS's "color interpolation"
+specification. This is how CSS computes which color to use in between two colors
+in a gradient or an animation.
+
+#### Deprecations
+
+A number of existing functions only make sense for legacy colors, and so are
+being deprecated in favor of color-space-friendly functions like
+`color.channel()` and `color.adjust()`:
+
+* `color.red()`
+* `color.green()`
+* `color.blue()`
+* `color.hue()`
+* `color.saturation()`
+* `color.lightness()`
+* `color.whiteness()`
+* `color.blackness()`
+* `adjust-hue()`
+* `saturate()`
+* `desaturate()`
+* `transparentize()`/`fade-out()`
+* `opacify()`/`fade-in()`
+* `lighten()`/`darken()`
+
+## Let Us Know What You Think!
+
+There's lots more detail to this proposal, and it's not set in stone yet. We
+want your feedback on it! Read it over [on GitHub], and [file an issue] with any
+thoughts or concerns you may have.
+
+[on GitHub]: https://github.com/sass/sass/blob/main/proposal/color-4-new-spaces.md#deprecated-functions
+[file an issue]: https://github.com/sass/sass/issues/new