How can a dark watercolour area be deepened without adding black?

Correct answer: Apply a transparent glaze of the same hue over the dry layer

A transparent glaze of the same hue over a dry watercolour layer increases the depth of the value while allowing the original colour to remain visible. Repeated transparent layers can create a rich dark without the dulling effect that black may produce when mixed carelessly. More water generally dilutes and lightens the colour; rubbing can damage the paper; and white raises the value and reduces transparency. The controlled sequence is therefore A.