As far as I know, lavender contacts would NOT show up well with dark brown eyes. Unfortunately, dark brown covers most colors. Maybe the extra bright ones *not sure what they're called* would, but I'm not entirely sure.
Also.. unless you need them for sight, I wouldn't wear them. For people who don't need them, all they do is hurt your eyes. I don't remember how exactly, but I DO know it wasn't a good thing. Wearing them just the one day probably won't do anything, but I still wouldn't recommend it. Having the character's eye color match is not that important(most cosplayers don't bother), and isn't really worth hurting your eyes.
Friday and Saturday, at Acen: Don't know yet. I may go as Rin Tohsaka again, and am contemplating Lisbeth from Sword Art Online.