It basically comes down to using linear brightness (at sufficiently high contrast between steps) instead of random brightness. TFA could get away with almost an

It basically comes down to using linear brightness (at sufficiently high contrast between steps) instead of random brightness. TFA could get away with almost any colors at all, even red to green, as long as it goes from light to dark. The problem is that they decided to use dark for BOTH ends of the scale, with light in the middle, so in the absence of color perception we can only tell whether a price is extreme or moderate.