I've not got a capsule wardrobe, but I think the idea is that everything mixes and matches, so you go for particular colours that will work together. I'd be inclined to go for separates over dresses so they'd be more versatile. If it was me, and I'm thinking about stuff I actually own here, which is probably not to everyone's taste, plus the time of year, I'd have:
Black skinny jeans
Black a-line skirt
Grey skinny jeans
Other a-line skirt, maybe a leopard print one
Dress that could be smart or casual, maybe a navy Whistles shirt dress
Stripy top
Turtleneck jumper - grey, I think
Black crew-neck fine-knit jumper
Grey crew-neck fine-knit jumper
Mustard cardigan
A couple of blouses, one black and white printed, and another more colourful one
Denim shirt
I think that would all work together to make different outfits, but I can wear smart jeans to work. The cardi and denim shirt could both go over the dress, and the stripy top could go under it (it's half-sleeved).
Could you look at things like stretchy tube skirts which will work over a couple of dress sizes?