My apologies. The "plastic purple thing" is actually the engine drain plug, which is metal, it's just really rusty (the car is now 12 years old...) and the purple hue is because I'm using an LED flashlight and the white balance of the photo is off.
It was actually a bolt, I BELIEVE a 10 mm (or maybe 12?). I attached a plastic hose (I used the same as for the radiator plug, it's close in diameter) and used a 10 (or 12?) mm socket wrench to open the plug (lefty loosy). I didn't have to completely remove the bolt; just loosened it enough to see coolant running out. Of course, you can only turn the plug about one ratchet click at a time because it's in such a tight location. And quite a bit came out! It's definitely worth draining if you can find it.
Hope this helps.