Basically yes, they were coloured in red so that people could see them from very far (just imagine that like todays stadiums it was very crowdy). It's a recent discovery, from 2015.

Here's the italian ansa about it:https://www.ansa.it/lazio/notizie/2015/01/21/colosseo-numeri-su-arcate-in-rosso_f23af60a-801a-409d-8cb7-4f967e0237bb.html