There is no such thing as no prep when you paint you'll just be fixing your own mistakes later i.e. more work.And you're right about one-coat paints but it's more half-truth than total myths. Some paints are marketed as one coats like Sherwin Williams Emerald, Valspar Reserve, Ben Moore Aura, Behr Marquee and a few others but two coats are always better. And while some colors can cover over dark ones with one coat, they always look better with two. And you'll never cover up like black or vibrant yellow with just white. And you'll never one-coat red, deep purple, or lime green unless you have a gray-background and even that's pushing it.