Although not an option, I'd put white for being at optimal level, yellow just below optimal, and green above optimal level. IMO yellow is for warning so it would seem weird to use it for good things. Green on the other hand gives the feeling of ease, making gear easy to wear, and as such fits the above optimal. White is very neutral color, so it fits well being in the middle.

That would put the scale into something like red>orange>yellow>white>green>blue>grey? which seems pretty logical to me.

//If I had to pick from the two though, I'd take green.