He may be just resting (or sleeping) so very little blood is flowing through his gills. Check on him throughout the day and see what color they are when he's active. Usually, the color of the gills will fluctuate throughout the day.
If he shows no other signs of stress, then it's probably nothing to worry about. Is everything else okay (like water parameters, temperature, appetite, etc)? That should give you an indication if something is actually wrong, or your axie is just calm.