Unwanted dark patches appear only where strokes overlap in a transparent layer. What is the main cause?

Correct answer: The paint layer became double in overlapping areas

The governing concept is cumulative transparency. In a transparent layer, each pass deposits another amount of pigment or tinted medium. Where two strokes overlap, the optical density increases, so less paper shows through and that region appears darker. The correct correction is to keep the stroke width, paint load, and overlap consistent, or allow the first pass to dry before adding another. Brush cleanliness, paper whiteness, and the colour name do not explain why only overlaps darken.