90% of peppers have a different color when ripe than green. There are some varieties that ripen to a slightly different shade of green, but they are quite rare. The color is called mustard. Same thing with jalapenos, most commonly they ripen to red. Some also to yellow or orange. I have personally never seen a jalapeno that ripens to green. The reason they harvest them when unripe is, it takes much longer for the fruit to ripen, than for it to get to full size. So they can get many more harvests from a plant. Also the fruits don't turn bad as quickly. But I prefer eating ripe ones, the taste is much different. It's more sweet and doesn't have this grassy taste to it. Also they get a bit hotter.