Back when I kept jewels, I just fed them some fry and they would turn red. Granted they must already have a good red bloodline but I watched many jewels go from that brownish color to red within a few days of smashing a bunch of fry or feeders guppies. They never reverted back to brown either.
Thanks, I'll have to try that! I know that when I first purchased the one I have now, it had a pretty brilliant color going on. But, as soon as I got home, it turned brown. Sometimes, it does display the red, but usually only when it's being aggressive. How often did you feed them the feeders?