For your colour theory yellow is usually electrical but...
Sizes are dimensions nominally in inches. Rating is for the pressure usually 13.5, 8.5 and 5 they are different wall thickness.
If you wish you can use grey for your drains if you can find the fittings. Blue is what everyone else uses. http://www.thaipipe.com/products&services/upvcpipe.htm
Stay away from HomePro and go to a local plumbing supply/hardware store.