Aluminum will oxidize, too unless it's clear coated. The center caps on my MSR wheels are aluminum and it didn't take long for them to start getting white chalky spots on them. Apparently they were never clear coated.
Your caps could be primed then painted to the color of your choice. i would recommend using a high temp engine paint then baking them at 250-300 for about 30 minutes.
We have 2 sets of knives that had a beautiful brushed aluminum handle. It only took several cycles through the dishwasher that they began to turn dark gray, then we started noticing they were giving off a black powder onto our hands. I scrubbed them down, rinsed them in white vinegar and let them dry. Then primed them with Zinc Chromate primer. When it dried, I sprayed them with a high temp engine paint, dried them, then stuck them into a board, put them in the oven, and they're still nice and shiny a year later. I do intakes and valve covers the same way.
