A UIColor is not mutable. I usually do this with colors, fonts and images. You could easily modify it to use singletons or have a static initializer.
@interface UIColor (MyProject)
+(UIColor *) colorForSomePurpose;
@end
@implementation UIColor (MyProject)
+(UIColor *) colorForSomePurpose { return [UIColor colorWithRed:0.6 green:0.8 blue:1.0 alpha:1.0]; }
@end