Согласно документации NSColor :
NSColor *myColor = [NSColor colorWithCalibratedRed:redValue green:greenValue blue:blueValue alpha:1.0f];
float red = 0.5f;
float green = 0.2f;
float blue = 0.4f;
float alpha = 0.8f;
NSColor *rgb = [NSColor colorWithDeviceRed:red green:green blue:blue alpha:alpha];