Assume an equilateral triangle on Color-Wheel. Combination of three colors at three corner of this triangle form a 'Triad Color Scheme'.
i.e. Orange Green Purple

Colors in this scheme should be applied very carefully, as colors involved in this scheme are not relevant to each other.

A combination of colors on Color-Wheel from Purple to Yellow-Green or any two or more colors from that range is known as 'Cool Color Scheme'.
i.e. Blue Green Purple

'Yellow-green' color in this scheme is known as 'Neutral', because it contains the exact same level of Warm and Cool colors. (50:50). While making a Cool Color Scheme, apply it with least quantity, if not used, color scheme can become more cooler.

A combination of colors on Color-Wheel from Yellow to Red-Purple or any two or more colors from that range is known as 'Warm Color Scheme'.
i.e. Yellow Orange Red etc.

'Red-purple' color in this scheme is known as 'Neutral', because it contains the exact same level of Warm and Cool colors. (50:50). While making a Warm Color Scheme, this color should be avoided or apply with least quantity.

In addition to basic principals of color & color harmony, how color perform in relation to other colors and shapes? is a complex part of color theory.

The middle green square is same but it appears different against different background.

Generally, brighter color bars seem larger than darker color bars of the same size.