One of the most popular and web-safe ways to create color in HTML and CSS is by using the hexadecimal color codes. From left to right, the first two characters represent red, the middle two characters represent green, and the last two characters represent blue.
Your 16 options for each character are the following:
F E D C B A 9 8 7 6 5 4 3 2 1 0
F is the strongest representation of the color, while 0 is the absence of that color. Play around with these colors until you get the colors you need.
So in order to get yellow (#FFFF00) is to add full red and full green. They cancel each other out, leaving only yellow. Also blue is set to zero so nothing comes in. The more blue you add here, the more the blue will begin to cancel out the yellow that is left until you finally end up with white! A little bit of subtractive color to mess with your brain.
Small tip: on a mac I use colourmod as a dashboard widget when I need to quickly find the value of a certain color.