The actual Rainbow Factory harvests colors using glass panels enchanted and then placed on the ceiling and the colored light is then caught in special artefacts that convert the light into liquid, which is then fed into vats and mixed vigorously by Pegasus workers.
So... Pinkie Pie did in fact put her hoof into liquid light and then lick it. Like licking a lightsaber. Lol.
I always thought that they shone light through mist. The light refracts, and the resulting colors are solidified with a spell, and then melted. So yes, it is liquid light. But a side effect of the process is that liquid rainbows reach over 9000 degrees calcius. Ergo, why Pinkie was hurt by it. The workers have a phrase for this: Taste the PAINbow.
their is a big problem with your theory. if in fact the liquid rainbow is 9000 degrees celsius then how is it that pinkie was able to pick it up in the first place. It would have melted her hooves off in an instant. I liquid rainbow does not get "HOT" in degrees but in spiciness