Not really sure why a rainbow would be an answer since rainbows are not matter. Odd.
Neither is fire.
Modern apologists for the Aristotelian elements note that they connect better with the states of matter: Earth-solid, Water-liquid, Air-gas, Fire-plasma. I'd suppose in thousands of years, people should have realized they were really talking about states of matter, when the noticed that water itself can appear as a solid, liquid or gas under conditions humans can experience without technology.
Meh. I believe it was Pliny who quoted some people who said clouds must be very dense, considering that thunder was clouds crashing together. I'm something of a firm believer that you can't really do science without technology. But whenever I say that, physicists and mathematicians want to bop me on the head.