Warms, Cools and Neutrals

Here's a quick exercise of a bowl and orange.  I used a limited palette of just Ultramarine Blue, Burnt Umber and Titanium White.  I mixed my own black by using Ultramarine Blue and Burnt Umber.  So, in-effect, I can say this is painted with just black and white.  By using a two-part black I can choose to mix a warmer black or a cooler black, or a neutral black.  I can make the shadows appear warmer by using more Burnt Umber, or cooler by using more Ultramarine Blue.  You'll do the same with the lights.  You'll learn to mix shadows cool and your lights warm.  Or, vice-versa: lights cool and shadows warm. 
Also note how the upper right corner is a neutral gray.  Don't forget that neutrals can be an integral part of the overall design (neutral: meaning, lacking color or an assigned temperature).  By having neutral shapes, you are taking the viewer's eye away from the neutrals and leading the viewer's eye toward the more predominant color shapes.   Learn to design your neutral shapes along with your warm and cool shapes.

Another useful exercise is to paint a still life made up of all white objects.  For example, a white mug, with a white napkin, on a white tablecloth, on a white table, with a white background... and so-on. This will push you to see the subtlety of hues within whites.  Yes, everything is apparently white... but what color white is it?  It will be some varying degree of temperature (cool-neutral-warm). 

This is a great way to develop your eye to see warm vs. cool, the extremes of their relationship and the use of neutrals.