They will slowly fill up the available space inside the husks. You can harvest when that happens. Sometimes the husks turn yellowish or light brown, indicating they are very ripe (and sweet) but some people prefer to prepare them more greenish. Other times they are so ripe they do fall off the vine, then pick those for sure. I find that on my vines the tomatillos ripen at much different times, this is nice so I don't have to eat them all at once.

cabrita,

Thanks for responding so quickly. I planted purple tomatillos, but even the ones that have filled up the space pretty well are still green. Should I wait for some color? Or did TGS send me the wrong ones?
Mine are coming in at all different times too, so it's good to know that's normal (and good that I don't have to make gallons of salsa all at once!).