@RGBLUM — Returns a luminance value for a color.

Syntax:
%@RGBLUM[color]

colormay be specified as #rrggbb or as a W3C color name.

If you specify the color in hexadecimal with a leading # sign, it must be either three or six hex digits long. If it’s three digits long, each digit is doubled; #8ac is the same as #88aacc. Alternatively, you can use a leading 0x and the rules above do not apply; the number of digits is not significant.

A luminance value from 0 (darkest) to 1000 (brightest) is returned. You can use this to pick a contrasting color. For example, if a background has a luminance of less than 500, white would make a good contrasting color for text; if it’s at least 500, then black would be a better choice.

The three color channels are given different weights: green is 58.7% of the luminance value, red is 29.9%, and blue is 11.4%.

rem  Is yellow bright or dark?
if %@rgblum[yellow] ge 500 ( echo It's bright! ) else ( echo It's dark! )


See also: the _BGCOLOR variable, which returns the current background color; and the @IMGCOLOR function, which returns a color from a graphics file.