In the ruleset it indicates 'The same color with different alpha channels is not considered separate colors.', isnt grey technically black but with a different alpha channel ?
Viewing post in Is it allowed to use shades of selected colors?
If you created a sprite using only black and put it on a background of white, using an alpha channel would enable you to create a full grayscale sprite with only two colors and an alpha, which is technically in line with the stated rules, but goes against the spirit of the rules. This was clarified in another post. Using sprites to get multiple shades in this way would be considered using multiple colors.