What is a likely cause if a dark band forms along an edge during colour filling?

Correct answer: A new wet stroke was applied beside a drying edge

The governing concept is maintaining a continuous wet edge during colour filling. If an earlier stroke begins to dry and a fresh wet stroke is placed beside it, pigment can collect at the join as the water moves and evaporates, creating a darker band. Therefore, option C is correct. Too little paint or small paper does not directly create this effect, while excess water usually causes paleness, flooding, or uneven spreading.