[nickjr]

It's officially "wood" xD you can see this in leader skills, active skills, and awakenings

"Green" is convenient, though, and I think most people tend to use RGBLD as the initials to minimize confusion, although sometimes people do use F and W if it's out of habit and/or if it's clear which monster is being talked about or something :P e.g. F/W or R/B Ame no Uzume, and GZL = Green Zhuge Liang, but B/G Sun Quan instead of B/W or W/G or--god forbid--W/W Sun Quan LOL

Interestingly, "yellow" and "purple" never saw popular use in place of "light" and "dark", unlike how "red", "blue", and "green" have been commonly used in place of "fire", "water", and "wood" for a long time. I did see some people call them them "white" and "black" instead, so maybe it's because of that? I dunno.