Of course you can switch colors. Just attach a new color the way you would a new ball of yarn in your current color.
Just be aware that if you have purl stitches on the right side of your work, both colors will show in those stitches. If you want a smooth transition, do the color change on a knit row, if possible. Some people don't mind the two-color purl bump. It's a pet peeve of mine, so if you don't care, forget what I said.