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Answer : Use rgba(255,153,128,1.0) instead of your hex value (though if that really is ARGB it's the same as #ff9980 in RGB - if you meant RGBA then you'll need rgba(255,255,153,0.5) ). the CSS3 spec says: Unlike RGB values, there is no hexadecimal notation for an RGBA value. so you will have to use the rgba(255,153,128,1.0) mentioned above.

Answer : Finally, I've found it! After thorough testing and experimentation my conclusions are: The correct way is to calculate maximum possible difference between the two colors. Formulas with any kind of estimated average/typical difference had room for discontinuities. I was unable to find a working formula that calculates the distance without blending RGBA colors with some backgrounds. There is no need to take every possible background color into account. It can be simplified down to blending maximum and minimum separately for each of R/G/B channels: blend the channel in both colors with channel =0 as the background, measure squared difference blend the channel in both colors with channel =max as the background, measure squared difference take higher of the two. Fortunately blending with "white" and "black" is trivial when you use premultiplied alpha. The complete formula for premultiplied alpha color space is: rgb *= a // colors must b...