If you want to change colors, simply use a square knot to tie on a new color at the beginning of a row. Depending on the stitch you are using, the loose ends where you tied it on may, or may not, show. If they do show, use a yarn needle to weave them into the stitches.
If you run out of yarn and need to tie on new yarn, be careful to match the yarn exactly. A brand of new yarn in the same color may not match exactly the same brand and color bought 2 years prior. Colors fade and can vary from skein to skein. Try to ensure you have enough yarn, purchased at the same time, to complete the entire project before you start knitting on your Knifty Knitter.