That is just how Veiled chameleons are.
Chameleons, Veileds mostly, will stay much darker because they are basking.
Dark colors better absorb that sun/light bulbs energy to warm up and digest food.
Veileds will almost always be dark during the day, unless they are placed in a very dense cage and are very high up.
If you would like to own a chameleon that is always beautiful looking, even when it's not sleeping I'd look into a panther.
The reason chameleons look so much brighter when they are sleeping is because they use contractions to control what pigments are shown in the cells.
When a chameleon is sleeping it cant consciously control what colors are being shown.
This why chameleons look the most spectacular(color wise) when they are sleeping of died.
The 3 types of pigment cells called malanophores, guanophores/iridophores, and and iridophores.
Malanphores are the deepest of the 3 cells and contain black pigment.
Guanophores/iridophores and in the middle contain blue pigment.
Xanthophores are the second outermost and have yellow.
Each layer of cells contains different colors and the outermost layer is actually clear.
This link goes into much more detail.
http://chameleonnews.com/?page=article&id=5
My guy pissed with normal colors: