Brightness weighted  This applies a brightness weighting according to the following formula, where ic = the calculated brightness value and exp the exponent, the function returns a
percentage that is then multiplied by 255:
var c = ic/255.0;
if ( c <= 0.04045 ){
return c/12.92;
} else {
return Math.pow(((c+0.055)/(1.055)),exp);
}

like PS  Approximates to the greyscale calculation formula used by
Photoshop. red_value × 0.2235, green_value × 0.7154, blue_value × 0.0611, each in turn passed through the inverse gamma formula:
var c = ic/255.0;
if ( c <= 0.04045 ){
return c/12.92;
} else {
return Math.pow(((c+0.055)/(1.055)),2.4);
}
then the
result vi passed through a gamma formula as follows, which returns a percentage
that is then multiplied by 255:
return vi<=0.0031308 ? vi * 12.92 : 1.055*Math.pow(vi,1.0/2.4)0.055;

W3C YIQ  YIQ coefficients also known as Digital CCIR601 and suggested in the Web Accessibility Guidelines from the W3C and challenged by me (see Holes in the W3C Colour Readability Guidelines. ) red_value × 0.2125 + green_value × 0.7154 + blue_value × 0.0721 
YIQ Sqd  sqrt((0.0.2125 * red_value^2) + (0.7154 * green_value^2) + (0.0721 * blue_value^2)) 
CIE XYZ  red_value × 0.299 + green_value × 0.587 + blue_value × 0.114 
XYZ Sqd  sqrt((0.0.299 * red_value^2) + (0.587 * green_value^2) + (0.587* blue_value^2)) 
Lightness  ½ × (max(R,G,B) + min(R,G,B)), see Puzzling Greys. 
Average  (R + G + B) ÷ 3, see Puzzling Greys. 