That's funny. It's pretty easy to grow green water. I'm trying to get rid of it in my tank. All you need is direct sunlight, put it next to a window and have the lights on 12 hours and nutrients, put some fish food a little everyday in there even if there is no fish. In a few days you will have a lot of algae. If you want green water, just keep the tank glass clean so that no other type of algae can grow accept the kind in the water. So scrape off any algae attached to the glass as they will compete with algae that are in the water.
It should be close to the temperature your fish can tolerate. I don't know how the algae tolerate cold water. My water starts out misty white haze. The water will start to get more hazy with each day and soon, before you know it, it's green and you can't see half way into the tank.