When your OC is literally an actual character but with different colors. These things are common because it doesn't take much effort on the design, but that's also why thy're shunned so often (especially when the creator didn't put effort in the process of recoloring either).
Because they looks so similar to the original non-OC character, the recolor is often a relative (most commonly a sibling) of the character. To hand-wave the fact that they've never met until now, they often refer the recolor as a "long-lost brother" of said character (which doesn't really make much sense considering they're separated for a long time yet they look exactly alike except colors).
The only reason why this kind of character is made is because the creator wanted to join in this OC fad but doesn't have the skillz to make a good character.