Why may making a new mixture be better when a shade contains so much black that the hue is unrecognisable?

Correct answer: Correcting an over-blackened mixture requires much paint and gives poor control

The governing concept is proportion control in pigment mixing. Black is a strong modifier, so once too much has been added, recovering the original hue usually requires adding large quantities of other colours. That wastes paint and makes the final shade unpredictable. A fresh mixture allows small, measured additions and clearer comparison. Black cannot simply be removed, and water does not restore a lost hue. Hence option D is correct.