he block on the left shows red, green and blue in brightnesses 255 192 128 and 64. The block on the right shows these converted to greyscale by the selected formula. This page is interactive, you can click in the radio buttons and checkboxes.

ote that some formulas are more convincing than others when looked at this way. The linear formulas (which includes Lightness and Average) are especially weak. The numbers in the greyscale boxes are the calculated brightness values.

And there are the print colours: Yellow, Cyan, Magenta, also in brightnesses 255,255 192,192 128,128 64,64 (e.g. yellow 255 is made up of 255 red and 255 green).