So wait, is it only the first set of leaves that are yellowish? Or the whole sprout?
And don't worry, I know what you mean about chilly, but don't forget that MJ likes temps around 72 - 78, so even if it does get a little lower than that on your thermostat, it won't be quite the same for your plant.
I live much higher north than you, and usually, our nights are colder than yours, I still wouldn't use a heating pad, because that just heats up the soil.
If the soil heats up more than the air, or even just to the same degree, it probably would cause root rot.
Oh, and dont start fertilizing it before at least 3 weeks worth of growth. Otherwise, it'd be like giving an energy drink to a baby?
So I'd highly recommend you don't heat the poor thing's soil.