For most peppers, once they turn red (whatever the ripe color is), I usually try to wait till they go a full day without getting any darker/deeper in color. There are exceptions; for instance, with yellow Bonnets, i tend to pick them once they are totally yellow, but before they go that deep Velveeta color.... And, with browns/chocolates/caramels, i definitely have trouble "reading" the colors, and tend to accidentally pick them on the early side...
That methods has worked for me, but it probably ain't the "best" way. I just tend to pick impatiently, so trying to wait a day had helped me allow fill flavors to develop.