You can pick your costume first, and then get a matching pair of Halloween contacts.
This way you can narrow down your field of choices to a couple of special effect contact lens ideas, and it won't be as overwhelming to choose. For instance, say you were dressing up as a vampire, well naturally and popular-wise, you would go for some kind of red crazy Halloween contacts.
You can pick your Halloween contacts first, and then decide on a costume.
Say you just love red contacts and know this is the color you want, or maybe it's a certain red style, then you think what costume would go with red eyes. So say you picked out the very popular White Out crazy Halloween contacts. Well, with these you could dress yourself up as a vampire, zombie, or witch.
The cool thing with contacts for Halloween is that your eyes are wearing the costume.
So whatever costume your body wears, your eyes have either the same costume, or an entirely different one. It's up to you how matchy-match you want to be with your costume and contacts. Unlike the faux pas of wearing no white past labor day weekend, when it comes to Halloween and crazy Halloween contacts, anything goes!