to add a new colour for a strip it is as simple as holding the new colour yarn as if you had been knitting with it all the time and just carrying on knitting with it for as many rows as you want. You can either cut the yarn each time you change colour, or you can ‘carry’ the yarn, which involves taking the yarn you are not using vertically up the piece at the edge or start of the round, twisting it around your working yarn every so often to keep it tidy.
The bits which hang down from where you add new yarn (often called ‘tails’) can be woven in after you have finished to secure and hide them.