These wave bottles can be made with any immiscible liquids. My students
usually use charcoal lighting fluid with water. Once your students have
gotten used to the colored layer being on the bottom (due to the greater
prevalence of water-soluble dyes) bring in one using colored lamp oil
instead of charcoal lighting fluid - or color both layers this way. An
additional teaching idea is to have one filled completely (no air
bubbles) and another with significant air space to show how one acts
like Italian dressing while the other doesn't mix very well. Lots of
"spare time" learning goes on this way