Calculate contrast from two RGB colors.

contrast.js

function contrast(c1, c2) {
  v1 = Math.min(Math.min(c1.r, c1.g),c1.b) + Math.max(Math.max(c1.r, c1.g),c1.b) / 2;
  v2 = Math.min(Math.min(c2.r, c2.g),c2.b) + Math.max(Math.max(c2.r, c2.g),c2.b) / 2;
  return Math.abs(v1-v2);
};

example.js

contrast({r:0, g:0, b:0},// black
         {r:128, g:83, b:83}// light-brown-red
);
// 147