Dividing the colour wheel in half, separates the warm, active and exciting colours, from the cool, passive and calming colours.
Warm hues reflect more light than cool hues. Warm hues are sometimes known as advancing colours, while cool hues are known as receding colours.
Warm colours can broadly be classified due to their yellowness or redness and cool colours can be classified according to their blueness. The process of mixing warm and cool hues means it is theoretically possible to have a cooler or warmer version of a hue, depending on the percentages of warm or cool hues in each colour.
Adding white to a pure colour will dilute its character and render it somewhat cooler.