GF can change color as they grow up. Sometimes you get a little orange one with white edges of the fins and the whole fish can change and the trim color disappears and then it ends up being a different color.
Panda GF which are black and white when they are young can turn all Black, or black and orangish, any number of changes are possible. As long as you are keeping water parameters in safe one, and are checking to make sure of that, and the fins have nice distinct shape as they are supposed to (not ragged or turning to white infected on the edges fin rot, etc) the color change shouldn't be much of a worry.
Also make sure you are feeding high quality foods.
Also plenty of veggie type foods or a special blend for GF. They love fresh plants and Duckweed, peas and other veggies like blanched Zuchinni. Sometimes they need to get a few trial bits/days to learn how tasty they are. Then remove at the end of the night or after a few hours any which isn't eaten so you do not end up fouling the water.