Mask is interesting in shape, to say the least. Not sure how well it'd be pulled off in minifigure form, but then again we have seen some intricate designs before with helmets and headdresses, so we shall see.
Whilst I do agree the colors are perhaps a little more on the vibrant side for what I typically expect from a Makuta, I do believe like for the mentioned villains Zarkon and Magneto, they could work... but perhaps in a different approach than pictured.
The Toa figures all have intricate detail printing on their torso and legs, which not only provides a little more visual interest, but also spreads out all of the colors featured rather nicely. I know I can't see the full torso from this artwork, but perhaps that's what this needs, to tone down the bright Metru Red (I see it as Metru, it's dark red) a touch. Incorporate more of the dark grey/gunmetal into the design around the symbol of darkness, perhaps feature a view 'veins' of purple power running to and from it, and that might will do the trick.
Mind, this is all just my personal opinion, but perhaps a better idea of what you're going for could be achieved with a full figure draw?