Some peppers are usually picked green - or so it seems, as we certainly see green hot peppers in the grocery stores.
But I believe they will all eventually ripen to some other color: unless they rot first.
Peppers (in the humid northeastern USA) tend to rot or be attacked by insects before they really ripen - at least this has been my experience. So I generally pick some peppers green and leave some to ripen - hedging my bets.
If you want to pick them green, you can judge by the size - if they seem large enough, go ahead.
If I am mistaken about this, will someone please correct me?
Pat