Cheap spray paint works for plumbing parts, I personally prefer flat or matte finis as it seems to adhere better. Let any paint cure first.
I actually Gel Stained the back of my tank to match my stand, it looks sharp.
A great trick I learned in painting the back glass without worry of salt creep or scratches from equipment destroying the paint;
Paint the back of tank, and let cure;
Using a disposable glove, IN A WELL VENTILATED AREA (outside is best), squeeze aquarium sealant onto a gloved hand and smear a thin coat coat of sealant over the entire back of tank. The sealant warms and thins from the heat of your hand and becomes the consistency of Vaseline, making it completely possible to spread a very thin coat.
If you are a perfectionist, or have nice tank trim you want to protect you could mask off prior to application.
When you're ready to change background colors, removal is pretty easy with a razorblade. Instead of dust and flakes, the paint comes off in strips and sheets.
I've been doing this for years and the paint on the backs of my tanks always looks as good as the day I did them, even after years of use.