If a mixture has become too blue what should be increased to make green?

Correct answer: Yellow

The governing concept is proportional colour mixing. Green is produced by combining blue and yellow, so when a mixture is excessively blue, increasing yellow shifts its hue toward green. Red would push the mixture toward violet or brown, black would darken it, and violet would make it more bluish. Therefore, option C is correct; add yellow gradually to control the final shade.