Jalapeños are generally green when they are unripe, but they can ripen and change color as they get older. Most people associate jalapeños with their green color, and you can certainly harvest and use jalapeño peppers when they are green. They will have a sharp, spicy flavor.
However, jalapeños can also ripen and change color to red or even purple, depending on the variety and growing conditions. When jalapeños ripen and change color, they often develop a sweeter and deeper flavor. Most people consider ripe red jalapeños to be less spicy than their green counterparts.