Bugfix: Use color.lower() for _color_dict keys

[psobolewskiPhD]

The _string_to_rgb helper function uses color.lower() to raise a ValueError, because the names are all lower case in _color_dict. However, in the next line, it still uses color as the key to the dict, which means that if the string is upper case, the first logic passes, but then a KeyError happens. This was first reported here in napari:
napari/napari#5599 (comment)

So this PR simply makes sure to use color.lower() as the key to the _color_dict dictionary.

All tests pass for me locally with this and the napari issue is fixed.

[brisvag]

Easy peasy!

[djhoese]

Looks good. Thanks. This logic makes me wonder if we should have some warning to tell the user that they are using a different name...but maybe this is fine as a nice convenience.

[psobolewskiPhD]

I think adding a warning makes some sense, just as an FYI, even though the code then uses .lower(). Want me to add that?

[djhoese]

I'm thinking maybe no warning. Then at a user-level people are fine to say "Red" or "red". Maybe that's OK...but then maybe the docstring should mention this? Ugh it is such a small decision that I'm sure will bite us in the future.

[brisvag]

IMHO waring is unnecessary. Worst that can happen (last famous words...) is the user somehow checking 'Red' == 'red' and failing, which is a pretty easy solution.

[djhoese]

I think this is good as-is. Thanks @psobolewskiPhD!