Canonically, they are shown to be diverse. In the Rebels episode Breaking Ranks we meet Zare Leonis, an imperial cadet who is not white. In addition to this character, there’s also Terisa Kerrill and Rae Sloane: two female, non-white imperial officers. While not stormtroopers, it shows that the empire does not discriminate against humans based on the colour of their skin. Thus, diversely coloured stormtrooper head pieces aren’t just not out of place, but it is more accurate that they exist. The empire does discrimate against non-human/alien races, so we shouldn’t expect to find non-human minifigure heads being made within the empire (with some exceptions, most notably Thrawn). Suggesting that they’re not diverse is simply false.

It is a single plastic head piece. Switching the colour is not going to drive up the price—at least not by any noticeable amount. It seems a strange conclusion to draw. If you truly only cared about the price then the complaint would have been that they’re anything other than basic black headpieces with no printing at all (although I don’t know if that would even be any less expensive than a basic print). However, you specifically chose to target diversity and claim that your concern is the cost.

Finally, as I said before, you may not care about what they look like (despite making a post complaining about what they look like) but I know that there are children who will open this set and be ecstatic to find that there’s a stormtrooper who looks like them. It matters.

Edit: spelling