Depending on the specifics of how the uniform detects that stuff, it might not be a plot hole.
Imagine for a moment that the uniform detects it by determining what % of your theoretical maximum power you're able to output. If Tasha's max power is ridiculously high, then it might display as black because he's using 10% of that potential output. But that 10% is still equivalent to a "normal" White Class, so the uniform displays as black.
Of course that's all just BS I made and it has a plothole itself (How does it detect and know what the "maximum" even is?, but the author's acknowledged that something's off so it's gonna be a plot point later instead of some oversight.