This is the main reason why hahlis colors seem so odd to me, especially since her I'd colors coincide with other well elements.
1) They're supposed to be odd to get people's attention to want to buy the set, and
2) They actually aren't odd for her in that situation at all. The lime represents seaweed, undersea plants. And that's the year she goes underwater. And:
3) Again, those colors likely weren't coinciding with other elements, if memory serves, yet at the time. That was chosen later and probably specifically because sets including them had been made. I doubt the lime counts, but the mixed white and blue in one piece from some of the Inika pieces would thus be easy to MOC Toa of Lightning with. Even with blue and lime (although it's probably not the same green usually seen in Toa of Plants), having an element use the ones available in just one set makes MOCing a Toa of Plants easy for fans; they don't have to buy as many different sets to do it. So it's really the other way around; the fact that existing sets match color schemes of the non-set element colors is to be expected, and it's an intentional choice.
To blue being under Fire range of colors, yes, Greg confirmed that. That is why Takua's blue mask wasn't seen as prohibitively strange; it was associated with blue fire. There would be a handful of other Ta-Matoran with that color somewhere (eyes probably most likely), we just never saw them, and they might not be from Metru Nui specifically even. But Vakama would have known this and would teach the known ranges to the Ta-Matoran, so they could choose replacement masks from within that range.
Had Takua become a Toa of Fire, from Vakama's perspective, the blue probably would have been natural to be mixed into his color scheme; picture a sword of fire, for example, starting blue at the base, then white, then yellow, orange, red. Then maybe something similar to that but just with blue, yellow, and red, from mask down like his Matoran form. Vakama might have even imagined such a thing happening when he thought Takua's collecting the six Toa Stones would Toa-ize him, not knowing he was Av.
But red and blue representing fire going underwater seems more unlikely. Incidentally, though, didn't Matoro's color scheme do something unusual too, with trans-blue or something? I'd have to check, but that might symbolize melting ice or underwater ice. Edit: No, I guess that was just the Inika form. But then, at least that could symbolize being charged up with lightning (trans plastic often indicates glowing since you can put a light behind it to make it seem kinda like that, like the eye plastic).
Edited by bonesiii, Jan 08 2014 - 10:29 AM.