I recently came across mention of a tequnique called tapestry crochet. This turned out to be a method of creating multicoloured patterns by having two or more separate threads and swapping them in and out, putting the unused threads in to form a filler.
I was interested by this as it is a more complicated multicolour version of the technique I have always used to create cats' doormats - mats that sit just inside the cat flap. Towels are inclined to slip and ruck up so I make crochet mats, crocheting around a core of thread to provide a strong, stable mat that stays in place and is easy to wash. It is startling how dirty the mat gets, especially in wet weather - and all that dirt on the mat is dirt that isn't elsewhere in my flat!
I am using left-over bedspread cotton here, working the crochet stitches using two strands and carrying 5 strands for the filler.