Hi and welcome to the forum
Besides what Fluttermoth has mentioned, fish have different colours and patterns for day and night. Many fish go darker at night and lighter during the day.
Turning the aquarium lights on can stress the fish if the room is dark, and they can show different colours from this.
In the morning you should open the curtains or turn the room light on about 30minutes or more before turning the tank light on.
In the evening turn the room light on, then turn the tank light off. Wait at least 30 minutes before turning the room light off.
If you have several lights on the tank then do the above but only turn one tank light on, wait an hour or more before turning the other tank light on. At night turn one tank light off, wait an hour before turning the room light on and turning the other tank light off. Then wait another 30 minutes or more before turning the room light off.
This allows the fish to wake up more gently instead of going from a dark tank to a brightly lit tank. And at night it allows the fish to settle down for the night in low level room light before the room goes completely dark.