Nils Gehlenborg & Bang Wong discuss “Mapping Quantitative Values to Color" in this month’s issue of Nature Methods. The article is paywalled, but I was able to access the figure without a subscription. They map out a systematic approach to color choice, starting with considering the salient regions of your data range and any values with special meaning (i.e. zero, 32 degrees Fahrenheit or sea level). They make explicit two options for mapping gradients to values:
…we can translate the ends of the color gradient to (i) zero and the theoretical maximum value or (ii) the observed minimum and maximum. The former approach allows us to interpret the data in the context of the theoretical data range (Fig. 1a). However, if higher contrast is needed from the graphical representation and zero is irrelevant as a reference point, then it is reasonable to map the lowest observed value to the lightest color and the highest observed value to the darkest color (Fig. 1b).