Swirl Without End is a good design for using up left over odds and ends in four colour groups. The original was made with two strands of DK; when one strand ran out another was added so the colours blend and there are no obvious stripes.
You do not have to use two strands. Any yarn will work. The afghan/rug may be larger or smaller, according to the yarn you have chosen but instructions are included for changing the overall size, if necessary.
It is worked in garter stitch, so always lies flat. It is made entirely from triangles. The curves are an optical illuion.