pleasing effect

according to the color emotion studies by Li-Chen Ou at the Department of Colour Science, University Leeds, UK.

Equal-chroma. Any two colours identical/similar in chroma tend to appear harmonious when combined together.

Unequal lightness values. Small lightness variations between the constituent colours in a colour pair may reduce the harmony.

High lightness values. The higher the lightness value of each constituent colour in a colour pair, the more likely it is that they appear harmonious.