When adding a new color, never knot! Instead, join new yarns neatly and invisibly at the beginning of a row. Knots have a way of coming untied and working themselves to the front of your work when you least expect it. We recommend this method: Place the needle into the stitch, dropping the old yarn and picking up the new yarn, leaving a 4-inch tail to be woven in.
best, clearest video for making a Russian Join. I've used this several times. You may need to have a few different size tapestry needles as you want a needle that is slightly smaller in diameter than the yarn you are working with.