When you start dealing with CSS, you need to learn about hexcodes, but they aren’t as intuitive as you might think. This is an excellent article that goes over the basics which will make working with hexcodes much simpler.
Here are some of the basic ideas from the article:
I think the most confusing thing is the letters. Once you realize that a through f are just added on after 9, it makes it a lot easier to understand.
Once you think you’ve got the hang of it, test your knowledge here: http://www.hexinvaders.com/