Just like bell peppers, jalapeños will go from green to red as they ripen. So you might be thinking that the heat will increase as the jalapeño ripens. That's not the case. Bell peppers get sweeter when they go from green to red, and jalapeño peppers follow that as well. So the heat will be the same regardless of if the jalapeño was picked when it was green or if it ripened until it was red. But, if you do get a red jalapeño, it will be sweeter than a green jalapeño. So you'll have the same amount of heat, but you'll have an extra bit of sweetness. And you might be looking for that in some recipes.