Computer generate or stamp "path" on white card stock
Dampen a small make-up sponge and brush along a rainbow ink pad (I used yellow-orange-red colors). Rub the sponge along the front of the "path" piece, and repeat until the card is colored to your liking.
Do the same with the larger card (I used more orange-red than yellow)
Glue "path" to red, then orange card stock, then to folded card.