Thank you for your question about the growths on leaves you encountered.  From your photo, these colorful structures look like galls from eriophyid mites.  These are microscopic creatures that secrete chemicals that can cause plants to grow little "houses" for their young.  Basically, they hijack the meristem cells, which direct plants cells in their growth.  The results are often quite beautiful, and most often do not do significant damage to the plant.

Here's a publication by OSU Extension that details different kinds of eriophyid mite damage.
https://bygl.osu.edu/node/1646

Here's one from Virginia Cooperative Extension that speaks to controls:
https://www.pubs.ext.vt.edu/content/dam/pubs_ext_vt_edu/ENTO/ENTO-147/ENTO-532.pdf

Isn't nature amazing?