A stand-alone a is the base link color. It’s your default style that will effect all of your links. a:hover is how your links will appear when hovered over.
You’re going to see lots of link styles in your stylesheet” h2 a, h3 a, etc. All of those are saying, if the element h3, which is a heading element, is a link, display it as follows. Does that make sense?
To change the color, you need to know the hex value of the color you want. Try colorschemer.com/online/ if you don’t have Photoshop.