Hello;
This may well be nothing to worry over. The look of fish can vary greatly and may be normal. I have not kept a peacock so can not give species specific experience.
I currently have a pair of angel fish who often have distinct and bold dark stripes sometimes and then the stripes fade away a short time later. This can happen in one day.
I also find fish to be less colorful over a light color substrate.
Some species are much more colorful during breeding periods and, I think, also when territorial. I have noted that a more dominant fish in a school seems to be the more colorful or have bolder markings.
All that said it is possible for a fish to have faded colors from stress, parasites or some illness. Look for secondary signs including but not limited to such as not feeding, being lethargic or not swimming properly.
Good luck