A category on UIColor. Pass in an array of colors and it will return the closest color to the receiver.
In your Podfile, add this line:
pod "MTColorDistance"
pod? => https://github.com/CocoaPods/CocoaPods/
NOTE: You may need to add -all_load to "Other Linker Flags" in your targets build settings if the pods library only contains categories.
UIColor *red = [UIColor redColor];
UIColor *redish = [UIColor colorWithRed:0.9 green:0 blue:0 alpha:1];
NSArray *palette = @[redish, [UIColor blueColor], [UIColor whiteColor], [UIColor greenColor]];
UIColor *closestToRed = [red closestColorInPalette:palette];
You guessed it, redish will be returned.