Realistic tempered steel effect on aluminum
It's possible that the only option available would be to use anodizing of a single or multiple colors. There are many resources, especially the YouTube type that can advise you. One recent video that is clearly presented is "Easy Aluminum Anodizing - Wawa dog tag"

It's generally considered difficult to get uniform coloration, although the result in the video is quite good. In your case you may not want it to be particularly uniform. You can accomplish gradients by immersing the part in stages. The first section will be in the longest and have the deepest coloring, while later segments will be in for less time.

The reverse can be true too, in that you immerse the entire part and remove it in gradual stages.

Heat can't be used to accomplish the desired effect. It works great on some steel and on titanium, but simply melts aluminum.