There's oxygen in the forge. It helps the gas burn to make it hot enough to harden the steel, so it's a big challenge to get rid of all of it. Consequently, at those very high temperatures there is some oxidation that causes the discoloration you see. It doesn't happen at any temperature you'd subject the armour to during normal use.
All materials have their limits, but this alloy, at this temper and thickness provides an excellent combination of dent resistance and light weight. It performs very well against rebated steel or blunter batons and rattan. Plus, because it's stainless, it's very low maintenance.