There are some other coloring surface treatments for steel, steel bluing for example, or thin rust-like coatings that aren't friable, maybe even thin plating of shiny metals.
But from the context, you seem to want to turn the mass of metal, into a vibrant primary color. That may be possible ...as surely as we can have gold that's cheap to acquire, or lead that isn't heavy, or a steel bar no one can bend, that is to say, purely in the realm of fantasy.
Since you want to learn something about the concrete casting process, by a destructive analytical method, and not use it later for dimensional purposes, maybe you can use colored plastic fibers?