How can a light blue be warmed slightly while keeping it light?

Correct answer: Add a little violet or reddish colour of similar value

The governing concepts are colour temperature and value. A very small amount of violet or reddish colour shifts blue toward a warmer appearance, and selecting a pigment with a similar value helps preserve its lightness. Black would lower the value, more blue would normally reinforce coolness, and green would usually move the mixture toward a cooler green-blue direction rather than warming it.