How to convert colors from one color space to another?

I'm not sure how to automatically convert them, but to identify different color spaces you can get the CGColorSpaceModel from the color's CGColorSpaceRef:

UIColor* color = some color;
CGColorSpaceRef colorSpace = CGColorGetColorSpace([color CGColor]);
CGColorSpaceModel colorSpaceModel = CGColorSpaceGetModel(colorSpace);

Then you can compare colorSpaceModel with the constants defined in CoreGraphics/CGColorSpace.h. UIColor's getRed:green:blue:alpha works for kCGColorSpaceModelRGB, whereas getWhite:alpha works for kCGColorSpaceModelMonochrome.

Note that a UIColor that was created with colorWithHue:saturation:brightness:alpha: will actually be in the RGB color space.

In swift 3 we can directly use

 let colorSpace = uiColor.cgColor.colorSpace
 let csModel = colorSpace.model

to get the color space and color space model from a UIColor.