Step 6: Paint the heart with food coloring
Of course, we're only making look "realistic" in a comic-book sense. The colors won't really be much like a real heart fresh from a ribcage would look - but it will look like what most people expect a heart to look like.
Use purple for the ridged areas shown. Try to paint darker near the bottoms and sides, and lighten up on the tops. You can paint a little red on the top part as a highlight. Just don't overdo it and you should be fine.
Now go over each vein with red. Just a little should do it. Color some parts of the heart's surface with a bit of red too to bring out color.
Now, only a couple more things to do. Trace each vein with a little bit of pale blue. Don't do the whole vein - just enough to give it a tint.
Finally, color in the holes we made in the arteries with black.