Painting Metal Shelves In Refrigerator

There's a special primer that's used on Alum. it has a mild acid in it to etch the serface and hold, you can get it just about anyplace, I suggest that you use "Rustoleum" paint, it's a hi quality paint and gives a porclin like finish

I would do the following:
1. Make sure your metal shelves are very clean, as paint will adhere poorly if they have residue on them.
2. Sand lightly to create a "tooth" for new paint to grip onto. (steel wool suggestion is good). Make sure you wipe off any shavings and dust.
3. Apply a rusty metal primer. You can find this at a hardware or big box store in either spray paint or oil-base brush on paint.Even though you didn't say your shelves were rusty, the mottling you described could be indicitive of corrosion.
4. Choose either a metallic-look spray paint to simulate the aluminum or else a white gloss to simulate enamel coated racks.
5. Sealing with a clear gloss may or may not be required dending on the durability of the paint you used.
NOTE: Make sure the paint you use is exterior as the inside of your fridge gets cold and wet...not unlike the the elements outside.

Lightly "sand" then with fine steel wool so the paint sticks. Spray paint and seal with several coats of waterproof satin "gloss"