changing colour from dark to light can mean a fish is sick or something wrong with your water,as its a sign of how the fish is feeling. 1st you will need to tell us what your water readings are. if you dont have a test kit please get one ASAP and test water for ammonia,nitrites,nitrates ph,then post results. almost every time you ask for help on this site we will ask for this info as it tells us a lot about how the fish is kept. we will ask about tank size fish & size and filter used. also if you include your maintenance program could be of a lot of help.
I may seem a lot but its the only way for us to give the right advice. you will soon learn that testing your water tells you a lot about your fish and the conditions its kept in. if you can get back to us with this info we will do our best to help.
PS. I just noticed you are new to the site.... welcome.