You should just dress however you feel comfortable. If you really want a 'look' dress like the bands you like.
Emo is a messy definition which covers a myriad of styles. I mean Panic! At the disco are so emo it hurts a little and they dress in a turn of the centaury manner. My Chemical Romance go for the dead parade band look at the moment and military spin in red and black. The Used go for a post grudge 'I've been wearing this for two weeks I think it's a white t shirt but I can't remember,' look.
Then you've got your insipid will I ever get the girl? Dashboard Confessional look, all sincere haircuts, jeans and unknown band t shirt. Sometimes they can be mistaken for awkward Maroon five fans.
Also there's the hilarious 'I want to get a mohawk but mom won't let' me semi post hardcore look. All black with red and purple stripes, band patches and badly applied eyeliner. Black is not optional here.
Oh yeah and there's the more metal end of emo with Thrice and Avenged Sevenfold - not what I'd call emo but y'know it's a pretty contentious issue. Thrice look pretty normal, dark genes hoodie maybe a wristband thrown in. Avenge sevenfold have the no arms vest, black nail polish, tattoos ' your mum is not going to be happy we're going out' look.
So you see, what do think emo is? I've yet to hear two people agree.
Anyway the point of all this is be yourself! It doesn't matter how you dress or whether your myspace page has 5000 friends.
Oh I'm so emo sometimes...
If all else fails here's a diagram
http://tn3-2.deviantart.com/fs10/300...y_KariNeko.jpg
This is just funny
http://lansing.ucoweb.org/files/emo.jpg
EDIT
Here's agreat site look at how all the bands dress, not that much black really more browns...
Ten Essential Emo Videos - That's Entertainment with The Express-Times