I've always been fascinated by the differences in the two sides of a butterfly's wings. It's hard to capture that since when the butterflies are still they have a habit of keeping their wings either open or closed - so I can only see one side. But here are a few that decided they would allow photos of both sides of their wings.
Dead leaf butterfly (kallima inachus):
Blue morpho butterfly:
Cairns birdwing butterfly with just a glimpse of the top side of its wings: