There's nothing to be fixed here; the Unicode/Emoji standard doesn't describe a colour for the folder emojis. Yellow/orange is the standard, yes, but Apple emojis are also grey, and other sets like Twitter's use blue.

See here: https://emojipedia.org/file-folder#emoji

Although if this is the time for opinions, I must say that I personally find the OpenMoji's grey folders a little bit… off-putting.
There's something about them that doesn't look well to my eyes.
Maybe other colours combinations like yellow/orange might look better (at least to my eyes) but I haven't done any tests so idk about that