What's the evidence that the colors were added later?

[Jorge_Stolfi]

And again don't know if it was discussed but here and for example You are not allowed to view links. Register or Login to view. there are some parallel "stripes". I am not sure if the artist:
- didn't know it is a rainbow
- didn't care to colour it properly
- lacked paint or time
- other

It is the other way around... In the original pen drawing, those were not rainbows but water conduits, as indicated by the water flowing out of their ends.  It seems in fact that each "rainbow" is two parallel tubes (or maybe two channels, or may be a single open conduit with a square "U" section.)

The Painter, for some reason, decided to paint one tube green and one tube yellow.  Then we compounded the confusion by interpreting those arches as rainbows...

By the way, that is one case where we can see some ink drawings (water flows into the pond) that were all but erased by the green paint.  The pool also looks like it should have contained something else besides green water...

All the best, --stolfi