It's a good question. I actually found a few other versions of this exact comic, first one is entirely without colours.

Second one is still black and white but has that watercolour feel to it, like in OP's version here.

Alternative black and white version I found on Rddt.

I don't have dates for any of these :/

It was rare in black-and-white comics to make someone dark-skinned because dark patches in general tended not to ‘read’ well in that medium. (This is the same reason Hobbes eventually lost the pads on his front paws.) And if I recall this comic was first in newspapers in black and white.

So I suspect that whoever colored it just didn’t know his jazz history.

Body and soul is indeed a jazz standard but heart and soul is a far more common song that every 8 year old learns. You are now subscribed to joke explainer