Schooling fish respond to the brightness of their surroundings.
In a bright area, especially light substrate and bright light they will lighten their own colors. They are trying to match the surroundings so they are less visible to predators.
In dark surroundings, dark substrate, some floating plants, they will make their colors stand out better. They are hidden from predators and want to communicate with their school.
A few things to try:
If you think it is the spectrum of the light you could try some other bulbs, other fixtures. Things like drop lights, bedside lights... whatever will hold other sorts of bulbs. Just to see if a different spectrum may help. Many fish will reflect more of that glowing shininess when there is more blue in the lighting.
You can see if the fish think your current lighting is too bright by shading the bulbs. A piece of window screen or similar. Something that will cut out 25% or so of the light.
You can see if the fish would respond to more plant cover by putting a piece of cardboard under your light.
Try any of these things for several hours and see if the fish respond.
I am not sure how to try darkening the substrate temporarily. Covering it with Black Diamond, or Tahitian Moon Sand will sure make it dark!