I thought it was a matter of detail resolution not simply color. My bad.

Misunderstood, didn’t realize you meant color

Heat stamping might be quickest alternative. Should be rugged and aid low-vision use.

Full brass letterpress type sets and die can be pricy, but the cheap leather iron kits at craft shops commonly include small metal alphanumeric types that could be set (clamped) and affixed to your stamp (burning/soldering iron or pliers + hot plate/stove).