you can use your skin.
it's the cheapest.
Next you can try apples, mellons and large leaves.
One can also image w/ red blood or green plant life (perhaps alge)
Forget modern emulsions and do something that will take one's breath away.
Why do people insist on doing in their kitchen or bathroom for 29 cents what huge multi million dollar companies did in mile huge complexes/combinats ?
You can image with many things