Create an opaque UIColor that appears the same as another which is translucent

Question

This color

[UIColor colorWithRed:75.0/255.0 
                green:215.0/255.0
                 blue:100.0/255.0 
                alpha:0.5]

and this one, with different RGB values and no transparency,

[UIColor colorWithRed:165.0/255.0 
                green:235.0/255.0 
                 blue:177.0/255.0 
                alpha:1]

look exactly the same on a white background.

How do I go from the first UIColor to the second UIColor?

Answer 1

Well, the answer will rely specifically on knowing the background color. Something like this would work, assuming backgroundColor and sourceColor are UIColors:

float blend(float src, float srcAlpha, float background) {
  return (background + src * srcAlpha) / 2;
}
UIColor *result = [UIColor colorWithRed:blend([sourceColor red], [sourceColor alpha], [backgroundColor red])
                                    green:blend([sourceColor green], [sourceColor alpha], [backgroundColor green])
                                    blue:blend([sourceColor blue], [sourceColor alpha], [backgroundColor blue])
                                    alpha:1.0f];

While the numbers aren't equal to your example, this is a reasonable blend function that is generic. Without more examples it's hard to know what the curve of your example is. The blend function you're using appears to use a different ratio for each color component, or perhaps your measurement of the resultant color values are being adjusted by the color profile of your screen.

Answer 2

NilObject's answer was good, but I ended up using something more specific to my problem.

The base color I wanted different shades from was this:

cell.backgroundColor = [UIColor colorWithRed:75.0/255.0 green:215.0/255.0 blue:100.0/255.0 alpha:1];

Here's my solution:

cell.backgroundColor = [UIColor colorWithRed:1 - (255-75)*rate/255.0 green:1 - (255-215)*rate/255.0 blue:1 - (255-100)*rate/255.0 alpha:1];

Where rate would replace alpha