One of my guppies always loses colors? (cont. in the question)?

After i wake up or get back from somewhere i see one of my guppies is losing their color. (black turning to grayish) Its about feeding time so i feed it regains the lost color. How can i fix that? It doesn't seem like its stressed the water parameters are perfect (0ppm ammonia and nitrite 30ppm nitrate) the temp is all normal for tropicals. The other tank mates are 6 guppies and 2 ottos. Its been 2 months and it just started. Help and Thx a lot!