As indicated, even when they are matched or ‘hit’ by another effect, the permanent supports do not change color anymore
I noticed this too but I think it was only one node for me. Both supports were red and my opponent was Koth. Not sure if those are relevant details, but it was a kind of annoying coincidence.
Would be fun to test if three perm supports in a row of the same color would go on forever. But that needs so much luck (nb2 ult can do it, but all need the same color as well)
Yes, it’s probably limited to one node. I’ve seen the permanent nodes change color afterwards during TGT.