A watercolour mixture looks correct when wet but becomes too light after drying. How should this be checked beforehand?

Correct answer: Paint a test swatch on the same paper and let it dry completely

The governing concept is the drying shift of transparent watercolour. While wet, water makes the mixture look darker and more saturated; after evaporation, the pigment settles into the paper and the visible value often becomes lighter. The reliable check is to paint a small swatch on the same paper and let it dry completely before judging or adjusting the mixture. Looking only at a wet palette is misleading, adding black changes the colour character, and folding the paper has no useful role in predicting the final value.