I don't think there's any value to that though, emojis were more neutral before they added a color skin to it. Instead of reading "I'm happy", now you have a combination of "I'm happy but also white" or "I'm happy but also black", I don't think that belongs here and I don't see any point to that.

If you don't like how they appear, some system setting could be made for changing the overall colours to switch them all to black, white, green or any other colour.