You could get these:
Neon Orthodontic Elastics
For more colors, you could just stack the colored bands in different combos. To identify which bottle they go with, just band the bottles too (they stretch about 10 times as big). Carto diameters are a little shy of 3/8", so maybe go a little smaller (like 1/4" or 5/16"). You can get them in either medium or heavy (thicker/stronger) from eBay for around $5.50 for like 200 of them, and they're reusable (haha, until they snap, but they're pretty tough). Nothing to peel off cartos, no expensive label refills...but you'd still need to remember which flavor's which color.
As for me, I usually stick to one flavor until it's time for a new batch of cartos, then I switch to another one. Easier for me to refill that way. One time I put Reese's Peanut Butter Cup in Piña Colada! ICK!
