Get RGB value from UIColor presets

I my application I pass RGB color value to server. My app uses UIColor predefined values, like [UIColor grayColor], [UIColor redColor]. I know that I can use following code:

const CGFloat *c = CGColorGetComponents(color.CGColor)

but only for colors that are in RBG color space, however, [UIColor grayColor] is not.

Is there any way to get RGB values for non-RBG colors?

Thanks!