I couldn't get to sleep last night, so sat knitting, consequently I've finished the rainbow today. It's name in the book is "striped wedges". I had a trip into Lancaster today and bought a ball of violet wool, just standard DK to finish the rainbow properly. The photo has messed the last two colours up, but they are indigo and violet.
It's about a metre long, but very loose knit so will probably stretch.
I wouldn't mind doing this again in a thicker gauge yarn.
I woke with a start very early this morning, and was unable to get back to sleep, so my rainbow was finished in double quick time.