One of the most interesting parts of color, to me, is color symbolism. That some colors can mean completely opposite things, depending on where you are in the world. In the western world, white is the symbol of purity and marriage; in the eastern world it is the symbol of death.
It’s especially important as a designer to be aware of these sorts of meanings behind colors if you’re working on a website that’s meant for a certain area of the world, or if it’s meant to be universal worldwide. Lots of white on a bridal website for an eastern country may be a bad idea and not generate much in sales.
Even keeping in my mind “patriotic” colors of certain countries’ flags is a good idea. You don’t want to insult a client in one country by accidentally using the of colors from a enemy country’s flag. Ditto for colors that are of importance to certain religions.
I’m skipping over the basics of color theory since primary/secondary/tertiary colors, complements, analogous, etc are all pretty elementary. I’m more interested in learning about what colors can symbolize. Tapping into the psychology of what colors can make people think and feel in an instant.