Tapestry Crochet is all about keeping more than one strand of yarn on the go at a time, crocheting over the unused colour with the working colour. It’s just like crocheting over ends! You could work with as many strands as you think you can manage, I guess, but this pattern involves only two at a time.
You can make this beautiful bag using the Tapestry technique, using only UK DC. It’s very, very easy.