Color Logic Algorithm

Here is a theoretical explanation

And the algo in C:

typedef struct {
    unsigned char r, g, b;
} RGB;

double ColourDistance(RGB e1, RGB e2)
{
    long rmean = ( (long)e1.r + (long)e2.r ) / 2;
    long r = (long)e1.r - (long)e2.r;
    long g = (long)e1.g - (long)e2.g;
    long b = (long)e1.b - (long)e2.b;
    return sqrt((((512+rmean)*r*r)>>8) + 4*g*g + (((767-rmean)*b*b)>>8));
}

Here is pgras' algorithm in Java:

public double ColourDistance(Color c1, Color c2)
{
    double rmean = ( c1.getRed() + c2.getRed() )/2;
    int r = c1.getRed() - c2.getRed();
    int g = c1.getGreen() - c2.getGreen();
    int b = c1.getBlue() - c2.getBlue();
    double weightR = 2 + rmean/256;
    double weightG = 4.0;
    double weightB = 2 + (255-rmean)/256;
    return Math.sqrt(weightR*r*r + weightG*g*g + weightB*b*b);
}