After removing wallpaper with a steamer I want to paint it, but using the least amount of paint possible.
Is there a product I could apply on the wall before painting so that one paint layer only will do?
Use a primer, like Zinsser Bulls Eye 1-2-3 or any other interior primer. It's rather hard to get true one coat coverage, why do you want only 1 coat? Are you trying to use an existing paint leftover to match the rest of the room? If so, you can buy tintable primer to better match the final color. We had to to this in a bright pink bedroom, so we tinted the primer blue, applied one coat of primer and topcoated with 2 coats of regular blue semi-gloss; worked quite well to hide such a strong color. You might get away with only one coat if the tint is relatively close. Check your local Sherwim Williams, Lowes, HD et cetera to see if they do tinted primers. Sherwin Williams is usually the best bet for special stuff.
If you're main goal is to minimize the amount of paint you have to apply then a Primer/Paint combo is probably the way to go. It's hard to say though whether one coat will get satisfactory coverage though. You'll get the best results if you don't try to change the color from one extreme to another.