While I value the water quality of the Colorado, looking at this from a different, it may be possible to get watercolor paints that are non-toxic, likely such as those that may be used for schoolchildren. I'd then think that using a gallon or two of river or creek water to clean your brushes daily would create a minimal amount of wash water. To me it seems like the effect of pouring this slowly into the river so it gets diluted by the 8,000 to 15,000 cfs likely flowing down the Colorado should be pretty low, especially when compared to the urine produced by the thousand people peeing into the river on any given day.
If you were dumping oil paints or petroleum into the river, I'd say no way, but there must be non-toxic watercolor paints on the market out there and I can't see how it would be all that bad...
Nothing in the world is more yielding and gentle than water. Yet it has no equal for conquering the resistant and tough. The flexible can overcome the unbending; the soft can overcome the hard. - Lao Tse