How do I get serrano peppers to turn red off the vine?
Most of the chillie peppers we buy in the stores are green.
Green colored peppers are hot but the red peppers are hotter and they
Have the most flavor.
Is there a trick to getting serrano peppers to turn Red before they shrivel up?

You just have to allow them to ripen. Green is the color of an unripe serrano chile. As they ripen they will change to red, orange or yellow.

I'm not sure if there is a way to speed ripening.

Some vegetables ripen faster inside a brown paper bag that has been rolled shut for a couple of days. Might work if the pepper is close to maturity.