You lay a pale shape over a dark gouache background, and the new colour becomes unexpectedly muddy. The background may have looked dry, but the water in the next brushstroke can make it workable again. That behaviour is part of the medium, and it can be either useful or inconvenient.
Royal Talens describes its traditional gouache as reworkable with water after drying. Check the paint you actually own: products sold as acrylic gouache use a different binder and should not be treated as though they share every handling property.
Make a small reactivation chart
Paint three patches on spare paper and let them dry. Across the first, make one pass with a lightly damp clean brush. Across the second, use a wetter brush. On the third, make several gentle passes. Notice how much colour lifts or moves and how the paper responds.
Next, try a light-coloured stroke over a dry dark patch. Change only the brush’s moisture or the number of passes. Keep the paint and paper the same. The point is to discover a workable touch for your materials rather than to find a universal water ratio.
Use the movement deliberately
Reactivation can soften an edge or help adjust a transition. When you want a cleaner overpainted shape, use a deliberate stroke and avoid repeatedly scrubbing the same area. Rinse a brush that has picked up the underlying colour before it returns to a light mixture on the palette.
Letting a layer dry is still useful, but it does not make traditional gouache waterproof. If an area keeps becoming muddy, pause and test the next move away from the painting. More paint and more brushing can make the problem harder to read.
Label your sample with the paper and paint range. A handling chart is especially helpful when moving between traditional and acrylic gouache, or between papers with different surfaces. It gives you evidence from your own materials before the next important brushstroke lands on the finished piece.