It sounds like you are being a super thoughtful mom, and as another mom, I appreciate it!
Here are my thoughts on the general subject: Never understood the 24 hours after a fever rule --- as most viruses are actually contagious for 5 days.
This is actually a pet peeve of mine -- when the "24 hour" rule is invoked, as a cold doesn't usually have a fever component -- and is certainly contagious. So while it's easier to send kids back to school quickly (which a lot of parents do when a kid has a cold but no fever) -- it definitely will be making other kids sick == and other families will have to deal with childcare, missed work days, etc -- so I'm superconcientious about sending a kid (yes even one with a "minor" cold) back to school too soon.
All that being said -- oh my goodness -- her only Halloween party this year? I would definitely hope that by the third day of her cold she is a lot better and can attend. But if she's got goopy green stuff, no way. Play it by ear the morning of. If not == Halloween is not for another week or so - take her to a pumpkin patch, or other events in town that are kid friendly for Halloween, and she will feel part of the festivities.