I have a love/hate relationship with coral.
Coral has had a problem in the past (2000 or 2001 if I remember correctly) where it was graying out with prolonged time in the flame, but it hasn't been a problem for a while.
Yesterday I was working with the new mango coral and it did that to me. Yup, I was working on a large bead and it spent a long time in the flame. Are you having trouble with the mango?