What is the most likely technical cause if colour gathers toward the centre during wet-on-wet blending?

Correct answer: The paper surface is tilted inward

The governing concept is fluid movement under gravity. In wet-on-wet work, liquid paint flows toward the lower or depressed area, so an inward tilt can make pigment gather near the centre. Too little pigment affects strength, a dry brush cannot create this pooling, and an opaque underlayer does not explain the movement. Keeping the board level and controlling the amount of water prevents the problem.