Recently I saw soda-flavored gelatin in the Jell-O aisle. Crush-flavored, to be exact. I knew my nephews would be overjoyed to sample these, so I picked up a small box of each.
Grape would be great for making dark colored skulls for Halloween.
Here's a skull-shaped jello mold you could use.
I'd use this pumpkin Jell-O mold with the Orange Crush gelatin.
If the Strawberry Crush gelatin comes out as red as pictured, there are all sorts of nefarious Halloween-related uses you could apply it to!
Find Orange Crush Jell-O, Strawberry Crush Jell-O, and Grape Crush Jell-O on your local grocery store shelves or via Amazon.