What suppliments are you giving him?
Are you using Repashy Calcium Plus by any chance?
Well, you know how if you take a multi-vitamin/mineral tab, and a few hours
later, pee bright yellow?
It's the water soluble vitamins and pigments that disolved and came out.
Same with your cham. Some food and suppliments have water soluble pigments, like paprika concentrate in the above mentioned calcium plus.
These things can tint the urates yellow, orange, and even red.
Besides all that, as already said it could be dehydration setting in, 'fatty' area's, like around the eyes, will start to shrink and the eyes may look
"sunk in".
That will tell you your cham needs water right away.