This may differ in other areas of the world, but to me the color green signifies a sort of purity/unity. It's a "cool" color, as opposed to "hot" like yellow. Green, to me, would be a much much better choice for "optimal level". I would say do red -> orange -> yellow -> green -> white/grey. I don't know how many steps there are, but this is just kind of a standard progression of bad -> good colors. Hots indicate a sense of urgency/impulse/warning, thus hot colors should be used to indicate the negative end of the spectrum. Cools should be used once the negative threshold is crossed.