the color scheme is based on Umaraks new trans lime color scheme. Lava beast has Umaraks colors mixed in with its own colors.
When the beast leaks came out people speculated that the (then gunmetal/trans neon green) masks would be corrupted versions of the uniter masks. that the beasts would use to corrupt the toa and set them up against each other.
So I changed tahu's trans color to trans lime so it would match the mask trans color and made the rest of his body gunmetal.
If I had it my way the mask would still be trans neon green with gunmetal and the color scheme would have matched that.