ive read about BTAs lightening up perculas too, and regaining their color after being hosted by carpets. could not say mine did this. in contrary, the both percs have gotten more black, the smaller one more though, he now looks almost like the solomon island perculas. they look great in their RBTA.
i guess the difference between the two is, like stated above, determined by their genes. i feed 2 different foods that are supposed to contain astaxanthin and they look healthy! they have been in the RBTA for a year, and since then still have gained black area between the white stripes.
greetings, martin