Ohhhh shit, it just hit me re-watching. I think the reason for the unexpected color of the Beast Titan is due to the power of the Warhammer too and not just that it's hardened (maybe both?). The coloring does look like the hardening power and it's also reminiscent of the Warhammer, and this time it fully registered that it's also actually "generating the ammo" (like we know the Warhammer power can do) that it was throwing at the airships each time sparks came from its hand, since otherwise it wouldn't have had anything on hand to be able to throw.