I’m not sure that hot-water bottle covers can be associated either with Frank L Baum’s story of the Wonderful Wizard of Oz or even Harold Arlen and Yip Harburg’s song that was cut from the score after the first preview of the MGM film adaptation! I’ve always loved rainbows.

I’ve lightly felted this hotty cover to fuse the fibres, but you could use DK and knit more tightly to make a denser fabric.

As for colour order, we have a mnemonic in England which, given that I live in Yorkshire, might not be so kindly taken! However, if you’re interested, it’s this -

Richard of York gave battle in vain

i.e. red, orange, yellow, green, blue, indigo, violet

The only problem that remains is to remember which way up a rainbow is…