Lightly "sand" then with fine steel wool so the paint sticks. Spray paint and seal with several coats of waterproof satin "gloss"
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.
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 high quality paint and gives a porcelain like finish