algorithm - How to compare two colors for similarity/difference

Question

I want to design a program that can help me assess between 5 pre-defined colors which one is more similar to a variable color, and with what percentage. The thing is that I don't know how to do that manually step by step. So it is even more difficult to think of a program.

More details: The colors are from photographs of tubes with gel that as different colors. I have 5 tubes with different colors were each is representative of 1 of 5 levels. I want to take photographs of other samples and on the computer assess to which level that sample belongs by comparing colors, and I want to know that with a percentage of approximation too. I would like a program that does something like this: http://www.colortools.net/color_matcher.html

If you can tell me what steps to take, even if they are things for me to think and do manually. It would be very helpful.

Answer 1

See Wikipedia's article on Color Difference for the right leads. Basically, you want to compute a distance metric in some multidimensional colorspace.

But RGB is not "perceptually uniform", so your Euclidean RGB distance metric suggested by Vadim will not match the human-perceived distance between colors. For a start, L*a*b* is intended to be a perceptually uniform colorspace, and the deltaE metric is commonly used. But there are more refined colorspaces and more refined deltaE formulas that get closer to matching human perception.

You'll have to learn more about colorspaces and illuminants to do the conversions. But for a quick formula that is better than the Euclidean RGB metric, just do this:

• Assume that your RGB values are in the sRGB colorspace
• Find the sRGB to L*a*b* conversion formulas
• Convert your sRGB colors to L*a*b*
• Compute deltaE between your two L*a*b* values

It's not computationally expensive, it's just some nonlinear formulas and some multiplications and additions.