Add missing entry for SKINCOLOR_BUBBLEGUM in ColorOpposite()

Noticed that Lilac didn't have an opposite color; turns out all colors from Purple onwards have opposites that are shifted because Bubblegum's entry was missing from the table.

Picked signpost frame 11 since it's the darkest of the yellows in Popcorn (wouldn't have made sense for it to be grey).

Also this is my first ever merge request (🎉 🎉) so please let me know if I missed any conventions!