Programmatically choose high-contrast colors

Question

This should be a simple question, but I haven't been able to find a way to make it work.

Essentially, I have a silly localhost page that I use in my webdevelopment. When I am surfing between our development server and my local version of the C# code (redirected from the dev url via host file) I have been known to sometimes forget what 'dev.foo.com' points at - local or server.

So I created a page which will run locally as my default web page's default page, so I can easily identify my localhost from the server.

This page does a lot of things randomly (including generating a character's starting stats for D&D), including setting a random background color. I do this by generating 3 random numbers between 0 and 255, and setting them as the RGB value for the body background color in CSS.

Given the 3 ints R, G, and B, how do I determine R2, G2, and B2 such that the second color will have high contrast with the first? I like having the page have random background colors (it keeps me from getting used to the look of the landing page) but I also like to be able to read the text.

Answer 1

If you flip all the bits, you will get the "opposite" color which would be pretty good contrast.

I believe it's the ~ operator in C#:

R2 = ~R1;
G2 = ~G1;
B2 = ~B1;

Answer 2

"Contrast" is a loaded word. If you just care about being able to read the text, then one easy way is to work in a luminance-based color space like HSL, and pick foreground and background colors with big differences in luminance.

The conversion between HSL and RGB is well-known--see Wikipedia for the details.

If you're talking about actual color contrast, it's not nearly as cut-and-dried (there are a lot of perceptual factors that, as far as I know, haven't been reduced to a single colors space), but I suspect you don't need that level of sophistication.

Answer 3

I did something like this in a Palm OS application. This is what I came up with. It doesn't give you "high contrast" colors but it gives you a background color that's different enough from the text color to be quite readable:

  // Black background is a special case.  It's fairly likely to occur and 
  // the default color shift we do isn't very noticeable with very dark colors.
  if (backColor.r < 0x20 && backColor.g < 0x20 && backColor.b < 0x20)
  {
   textColor.r = backColor.r + 0x20;
   textColor.g = backColor.g + 0x20;
   textColor.b = backColor.b + 0x20;
  }
  else
  {
   textColor.r = backColor.r + ((backColor.r < 128) ? 0x10 : -0x10);
   textColor.g = backColor.g + ((backColor.g < 128) ? 0x10 : -0x10);
   textColor.b = backColor.b + ((backColor.b < 128) ? 0x10 : -0x10);
  }

You might not need to do black as a special case for your purposes - Palm's color handling is a bit funky (16-bit color).

Answer 4

There are some good resources (and algorithms) to address this at http://juicystudio.com/article/luminositycontrastratioalgorithm.php

Answer 5

You need a difference in brightness for text to be readable, as color vision itself has too low resolution.

So as an algorithm I'd suggest the following:

• Pick a random background color.

• Then decide whether it is a light or a dark color. For example you could check whether the average of the three primary colors is greater or equal 128.

• For a light color use black text, for a dark one white text.

Answer 6

Check out this PHP solution: Calculating Color Contrast with PHP by Andreas Gohr. It can be ported to any language of course.

He also has a very nice demonstration of his contrast analyzer where you can find some minimal contrast levels to work with.