The contrast primer seems like a great product for the first basecoats over white. Spray white, add contrast and then you're set for fast shading and highlighting. Seems excellent for fast but good looking paintjobs.
And yes you can make the stuff yourself, but I don't care for that...I'm way too lazy

. I want a simple and good product from a pot that is consistent. It's the main reason I love the old GW/AP washes. You can speed up your painting a lot and they are great for quick good blends. So I'll probably buy all of them!
And sure they work fine over white. No need to buy super expensive coloured primers. White is the way to go.