Fastest formula to get Hue from RGB

In addition to Umriyaev's answer:

If only the hue is needed, it is not required to divide the 0-255 ranged colours with 255.

The result of e.x. (green - blue) / (max - min) will be the same for any range (as long as the colours are in the same range of course).

Here is the java example to get the Hue:

public int getHue(int red, int green, int blue) {

    float min = Math.min(Math.min(red, green), blue);
    float max = Math.max(Math.max(red, green), blue);

    if (min == max) {
        return 0;
    }

    float hue = 0f;
    if (max == red) {
        hue = (green - blue) / (max - min);

    } else if (max == green) {
        hue = 2f + (blue - red) / (max - min);

    } else {
        hue = 4f + (red - green) / (max - min);
    }

    hue = hue * 60;
    if (hue < 0) hue = hue + 360;

    return Math.round(hue);
}

Edit: added check if min and max are the same, since the rest of the calculation is not needed in this case, and to avoid division by 0 (see comments)

Edit: fixed java error

1. Convert the RGB values to the range 0-1, this can be done by dividing the value by 255 for 8-bit color depth (r,g,b - are given values):

   R = r / 255 = 0.09
   G = g / 255 = 0.38
   B = b / 255 = 0.46
   

2. Find the minimum and maximum values of R, G and B.

3. Depending on what RGB color channel is the max value. The three different formulas are:

   • If Red is max, then Hue = (G-B)/(max-min)
   • If Green is max, then Hue = 2.0 + (B-R)/(max-min)
   • If Blue is max, then Hue = 4.0 + (R-G)/(max-min)

The Hue value you get needs to be multiplied by 60 to convert it to degrees on the color circle. If Hue becomes negative you need to add 360 to, because a circle has 360 degrees.

Here is the full article.