When we see colors we are actually seeing the reflected light of that particular part of the visible spectrum. White reflects more colors (that is why it is white) brown and black reflect the least colors of light and absorb the most light which makes them appear dark to our eyes.
In simple terms, the dark colored items heat up in sunlight because more light is being absorbed by the dark color and less light is reflected. White or light colored things tend to heat up less because they reflect much or most of the light. Other colors are in between these two (including gray).
We cannot see parts of the spectrum below the shortest visible light colors (violet) or above the longest visible light colors (red) and there are other factors that come into play for absorption or reflection of those shorter (ultra-violet) and longer (infra-red) wavelengths but in general White surfaces tends to be the coolest because white reflects the most light. Smooth surfaces are also better than rough surfaces for less absorption because smooth surfaces have less surface area.
I hope this information is useful.
Edited by ckent323, 29 August 2020 - 07:00 PM.