Decorative hard anodizing? 1

The color comes from a dye added to a protective sealer infused into the microcracked aluminum oxide surface that anodizing produces. Hard anodize is thicker than decorative anodize, but you still need to seal it, and you can still tint the sealer. Don't expect to get the exact same shade.

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Mike Halloran
Pembroke Pines, FL, USA

Hard anodize has some inherent coloration due to alloying elements in the aluminum. So it's doped alumina, a la ruby or sapphire. Also, some intermetallics and silicon form particulates (inclusions) in the alumina, so can get speckled effect. There is also some change with thickness. The natural coloration adds to any dyeing, so it is somewhat difficult get lighter colors on certain alloys. Using a sulfuric+oxalic acid (rather than simply sulfuric) gives a yellowish hue, as does dichromate sealing.
The Aluminum Anodizers Council's 'Anodizing Reference Guide' gives the natural colors expected for various alloys.
Kaiser Aluminum developed a number of 'Kalcolor' alloys which developed natural colors (amber/gold, bronze, black)when anodized. Alco had a similar Duranodic process which depended more on organic acid additives to the anodizing electrolyte to develop color. Don't know whether still available.
Most frequently, hard anodizing is dyed black or a darker blue or red. Some colors used in anodizing (the same dyes are used for both hard & conventional anodize):