What is generally added to dilute colour in a wash technique?

Correct answer: Water

The governing concept is dilution in a water-based wash. Adding water increases the liquid content of the paint, lowers its concentration, and makes the layer lighter and more transparent when spread with a brush. The exact amount controls the value and transparency: more water generally produces a paler wash. Sand, paper pieces, and wax do not normally dilute watercolour paint and would interfere with a smooth wash.