I’m knitting the “Hat full of stars” pattern hat from ravelry ( http://www.ravelry.com/patterns/library/hat-full-of-stars ), I’ve been told to cast on 176 stitches - 88 of each colour, the pattern only shows 88 stitches - is this because I’m essentially knitting a reversible hat?
Yes, a reversible hat where the inner and the outer hats are knit at the same time.
Love this pattern. Thanks for the link.
The video links are quite good for the cast on and double knitting.
I had a blonde moment and though double knitting was when you knitted with both left and right hands during a fair isle… 
How easy would it be to add an extra colour on each side - a light purple for the light side and a dark purple for the dark side? Presumably just one colour for either stitch in the stitch pair?
Cute hat! Yes, that’s how it works with double knitting. It’s kind of like doing the heel on a sock…magic!
I’ve never done that. I suppose you could if you wanted to use or carry the colors for the entire row. If you wanted the new color for only one star for example you’d still have to carry it for the entire row or else run into a problem similar to that of intarsia in the round.
You may also have to consider whether the hat is getting too thick between the two layers because of the extra yarn. Interesting idea though.
@salmonmac it would only be for the little ‘crosses’/stars so was going to tie off after each one - more ends I know but saves having to carry the extra 2 colours (light and dark purple) all the way up/round the work,
That would work and it’s an interesting addition, much easier to do than carrying yarn. Looking forward to seeing your hat, inside and out.