Iain
Wee salmon doubles, imvho, need to be sparse, almost anorexic.
I like them with squirrel wings, dyed, and I've got away with trout capes for hackles.
Small doubles need to "work" in the water and little fatties on light nylon don't really do that. My best pattern is a wingless Greenwell, more like a spider than a salmon double. My other favourite is a wee treble, but can be a double, in silver or gold with a sparse black wing. Definitely less is more. Think what tackle you will attach them to and go from there. A #7 or #8 line with 6-8lb nylon (or less if you're nuts like me)will make the fly swim much more attractively. My most memorable salmon was on an 11' #5 Clan trout rod on a #14 Greenwell spider, 5lb Drennan sub surface nylon!
At night!!
On Stoats Tails etc I used to tie the "hackle" as a beard rather than a full wound hackle so keep everything slim.
As for colours, orange, black, blue and yellow are the basics - that will cover Allys shrimps, Stoats, Marys, shrimps and so on. Think also about how slender you can make the body - too much heavy salmon silk can swell a good fly into a right porker, as can thick oval tinsel etc. Partridge wee doubles were the knackers back in the day and I still have a shed load if I ever need them again.
No doubt some salmon angler will be along in a minute to rubbish my theories, but wtf, all that worked for me