The upper part of a flat watercolour wash became dark while the lower part became unintentionally light. What is the most likely cause?

Correct answer: The amount of water increased with each stroke

The governing concept of a flat wash is consistency: each stroke should carry a similar pigment-to-water ratio and connect evenly with the previous stroke. If more water is introduced with each successive stroke, the mixture becomes progressively diluted, so the wash loses pigment strength toward the lower area and appears unintentionally lighter. A dry sheet or broad brush does not by itself create this gradient, and adding white is not a normal watercolour cause. Therefore, option A is correct; remixing consistently helps prevent it.