I have found too that having a UVB/A light helps brighten up the colors of the shell. It must be close to the basking platform though. Get the light as close to the platform as you can, but still be far enought away so the turtle can't touch it.

Also be sure the light has both UVA and UVB lights. In addition to brighter colors it seem as if my turtles are healthier and happy this way!
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0.1.0 ornate box turtle
1.0.0 eastern box turtle
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.11.0 chickens
1.0.0 Dutch(rabbit)
3.2.0 children (do I still count the married ones?)
1.0.0 husband