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

[Mauro]

I find it interesting that on You are not allowed to view links. Register or Login to view. there is a massive painting instruction that was obviously ignored. The most likely explanation to me seems that the painter simply forgot to paint it.

Another simple explanation is it's no painting instruction at all. It's surely possible for the painter to forget the only painting instruction in the whole folio (or both, if you consider the 'F' in the rightmost flower to be a painting instruction too), but it's weird.