If you're identifying color for pretty much anything digital, you're working in an RGB colorspace. If the project you're working on requires percentage representation, gray is made of 50% red, 50% green, and 50% blue.
If you're identifying color for a print project, you're most likely using a CMYK colorspace—the percentages are 0% cyan, 0% magenta, 0% yellow, 50% black.