Colour is one of the keystones of design theory. I think it is a mistake to think of it as the easiest to master – no matter how much you think you know, there are always surprises.
Sometimes going back to the basics helps reset our preconceptions and aids discussions about colour with clients. A good place to start is this handy cheatsheet from Paper Leaf which includes a colour wheel, colour relationships and subtractive vs. additive colour.
The last of those often confuses me as I work online and in print, I have to work in CMYK (subtractive) and RGB (additive). The RGB challenge game has helped me understand better what effect the different values of the pixels has on the appearance.
This article from Creative Market uses 20 infograms to show the role of colour on things like sleep, purchases, cultural perceptions and learning. The cultural aspects of colour is often used in branding as shown by this infographic from Iconic Fox.
Colour correcting photos is a central part of my job. With modern technology you can go beyond making images look bright and saturated, you can adjust the colours to change the entire feel of the image.
If you’re looking for a simple but visual way to pick colour sets with hex numbers try Color Supply.