Thanks for all the recommendations. I had wire-brushed away the corrosion from a couple of the THs when I bought the boat 3 years ago and it looked a bit pinkish, so I was planning on replacing them. However based on the feedback to this thread I got in there with a file and really got down to bare metal, and it's bright bronze - so I guess I'm OK! Seems there's just a surface layer of reddish corrosion.
A couple of them are weeping so while I had her out I cleaned out between the TH and the gelcoat and sealed it up with epoxy, so hopefully that'll fix the problem.
Appreciate the advice,