Use whatever you like! Having a connection with a tartan is obviously the nicest though. Flashes must certainly match, unless you use a solid colour that is the most present in your kilt. Or just plain red, which is what people also seem to use. Since you are a starter, I suggest buying coloured hose, although if you prefer white hose I'm not going to start a flamewar - also depends on your tartan to be fair. In response to the first post after yours, I do have to say it would be utterly devilish to wear pink flashes with a green kilt or so. For the love of no god, don't do that ^_^ especially during a prom, where your kilt and flashes will mostly be the most colourful, do match it with your kilt. I just saw the armstrong tartan - why not go for dark green hose with red flashes? so it brings out the stripe!

Jacket -- I wear a kilt to prom myself and I strongly prefer the argyll jacket. I just hate the overall look of the prince charlie style jacket (I do however really like the sherrifmuir jacket...) so I'd recommend that.

Furthermore, since a prom is usually a black tie event - wear a black tie. Unless you can find a rouche tie, if you really hate black bowties because they remind you of waiters (like me).

Also, don't forget to get yourself a decent sporran and kilt pin. I like it when my sporran matches the kilt pin in a fashion (if you go for a thistle pin, go for a thistle cantle as well!). When that's done, I reckon you're pretty much set!

Also, you going with a lady? consider getting a corsage using a bit of tartan, so she matches with you! Don't overdo it (no sashes) after all, she's the lady and she deserves all the attention in her own way, without you draping your trademark all over her - but that's my opinion!

Hope that helped!