Toyota's Forlife coolant is a pre-mixture of ethylene-glycol (aluminum friendly) antifreeze and comes in either green or red in colour -- there is no difference between the colors. It is possible to use other ethylene-glycol based mixtures but it's important to ensure that the correct quantities are used -- it's easier to just purchase it from Toyota.
Toyota recommend 14 quarts (2 galons) of Toyota Red. Toyota Red is aluminum compatible (Extended Life) ethylene-glycol; it's a very good coolant, and should last around 30,000 miles.
Whilst you could mix red and green ethylene-glycol coolants it would look a nasty color! However mixing ethylene-glycol and non-ethylene-glycol is not recommended.
Flushing the entire system is not a simple task, and if you don't remove all air bubbles, may result in an overheating engine. For complete flushing instructions refer to Pad and Wheels, or Detonated.