Two different products; Aluminum Black for the aluminium and the 44/40 for steel.
Noticing the raised lettering on the aluminum caliper, I thought it would be straight forward to darken it with the Burchwood-Casey product.
For steel, the 44/40 is great for touch-up so would likely do a decent job of dealing with chips and scratches, perhaps bolt heads. It works best on low alloy steels, but beads up and runs off of high chromium (stainless) alloys.