I've used RC glossy with kids watercolor. (You know, the kind where the paints are like oval bars of soap all lined up in a plastic case, and you dip the brush in water before selecting a paint)
Basically the gelatine layer will accept some water and the majority will evaporate. It will accept enough to define the brushed on area, and it will accept it slowly. Put on too much watercolor and it will spill out of the area you are trying to do. Let it dry by evaporation. If the coverage isn't thick enough color, let it dry or mostly dry, and add another layer of color to increase density. Don't paint adjacent areas with different colors at the same time, lest the wet watercolor might mix before it dries in place. Once it's dry you can also layer another color on top of an existing one as well.