When Serrano Peppers are green they're unripe which is when they are usually sold and when the Serrano Peppers are green and unripe they are usually not as hot as they would be when they turn red.
When Serrano Peppers turn from green to red that is when the Serrano Peppers become ripe and then when the Serrano Peppers are red they tend to be hotter than the green ones.
Still the green Serrano Peppers are also hot but in my opinion when I eat red Serrano peppers they are hotter than the green ones.
So if you want the hottest Serrano Pepper I would go with the Red Serrano Peppers.
Serrano Peppers register between 10,000 and 25,000 Scoville heat units on the Scoville Scale.