Electroplating/Anodizing is the term you're thinking of.

One thing I've done with a knife is copper coating.

I used a combination of copper sulfate from the hardware store (fungicide/algecide in blue crystal form) and a bit of salt and baking soda to make the electrolyte solution, a piece of copper pipe on one end of the wire and the knife blade on the other.

I should have done an intermediate nickel layer but I was experimenting and didn't prep right or electro plate super long

End result was a copper colored knife with a shiny steel blade edge. If I left it in longer it could have built up enough copper to polish it up.

To get a trans flag you would need to use a combination of different methods from electroplating to titanium vapor.

Supposedly you can electroplate aluminum onto steel, so you could simply electroplate the steel with aluminum, then do the normal acid dye for the anodizing the aluminum. Then a quick sharpening of the edge and you're good to go.

Regardless of the method, it would look fuckin rad