There is a great scene in Mad Men (S1 E6), where girls are asked to try out some lipsticks. Secretly, they are being watched by a couple of men through a mirror in opposite room. Joan (the red dress) is the only one among girls who knows that and she does enjoy it.
So while other girls are trying out the lipsticks, Joan entertains the men behind the mirror and drives them totally crazy, especially when she leans over and puts out a cigarette. Wow, that was sexy! Sexy is also her dress, but most importantly, the color!
The red color of her dress is by no means an accident. Costume design plays actually a big role in each episode and gets even analyzed!
I am completely aware, that this color may represent many other things (violence, danger, happiness, anger) and this largely depends for example from which country do you come from.
But in this case, I think that the color red represents the symbol of sexuality and desire. Here are other examples from the same episode:
- Roger seduces Joan in a hotel room.
- Don dates Rachel.