I’ve talked about this dress of Amy’s before, but to repeat myself…
Notice how the red in her dress calls back to Selina’s coral dress and the pink of the walls. Not only that, but everyone except Sue and Mike has a flash of red in their costume. It centres Amy and Selina as the focal points of the scene – everyone else is reacting TO them. (If I remember rightly, in Dan’s scenes at the lobbying firm everyone is dressed in black and grey, demonstrating how miserable he is. Colour doesn’t re-enter his storyline until Amy does).
With one exception. We can’t see it here, but Karen is dressed entirely in black – whereas Amy’s black is shot through with Selina’s red and pink. In other words, the clothes are telling us who Selina’s REAL lieutenant is, the person who has a genuine connection with her, and it’s not Karen. (It may not have been romantic, but Amy and Selina’s story in season 4 absolutely was a love story – and it makes me sad that it’s been so comprehensively trashed by season 6).
Amy really doesn’t wear patterns often, especially not large, abstract patterns like this one, and when she does, it tends to be a sign of…conflicting emotions. See the leopard print(ish) dress she wears for her date with Dan in 6.05, the patterned blouses she wears with Dan and Buddy, and the (gorgeous) black and green wrap dress she wore for her anniversary dinner with Ed (a dinner which, I think we can safely say, signalled the end of that relationship).