UIColor category to cross fade between two UIColors at a specific ratio
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UIColor+CrossFade is a UIColor category adding a class method to return a UIColor created by cross fading between the two specified UIColor objects at the specified ratio (between 0.0 and 1.0).

Using UIColor+CrossFade:

Import UIColor+CrossFade.h, and use as follows:

UIColor *colorA = [UIColor redColor];
UIColor *colorB = [UIColor blueColor];
UIColor *crossFade = [UIColor colorForFadeBetweenFirstColor:colorA secondColor:colorB atRatio:0.5f];
// crossFade is purple

At a ratio of 0.0f, the result will be fully firstColor. At a ratio of 1.0f, the result will be fully secondColor.

Remember - you control the ratio and it doesn't have to be linear! ratio = powf((1 - value/valueNominal), 0.333f) is valid as well.

There is also a convenience method that returns an array of crossfaded increments between the specified firstColor and secondColor (thanks peyton!):

UIColor *colorA = [UIColor redColor];
UIColor *colorB = [UIColor blueColor];
NSUInteger steps = 5;
NSArray *steppedColors = [UIColor colorsForFadeBetweenFirstColor:firstColor lastColor:lastColor inSteps:steps];
// steppedColors is an NSArray of 5 UIColor objects, crossfaded in steps between firstColor and secondColor

You can also use a version of the convenience function that allows you to define the curve the method uses when calculating your color steps as a block. For example:

NSArray *steppedColors = [UIColor colorsForFadeBetweenFirstColor:firstColor 
                                               withRatioEquation:^(float input) {
                                                     return powf(input, 1/4.0f);

See the included UIColorCrossFadeDemo project, which uses the extremely handy InfColorPicker, for further example usage.

If you use this in your app, I'd love to hear about it!