I misspoke. Oxides make red-purple-black.
"The cause of these colours is the usually trace oxide amount in the soil. The soil is usually composed mostly of silt-sized quartz. This quartz is coated with with various oxides. Differing proportions of these oxides will change the colour of the soil. Fe2O3 appears red, Fe3O4 is black and FeO(OH) is yellow. Manganese oxides (with either Mn2+, Mn3+, Mn4+, or mixed valence) can tint soil dark black, or slightly purple when mixed with iron oxides. Clays can also change the colour, with the famous example of green glauconite."
This is from Peru.
"Hue. The hue indicates its relative position on a scale of 100 different grades of color. Hue depends on the wavelength (in short: the real color). The notation is based in 10 major classes: red (R), yellowish red (YR), yellow (Y), greenish yellow (GY), green (G), blueish green (BG), blue (B), purplish blue (PB), purple (P), reddish purple (RP). Grayish, blueish and greenish hues may be grouped as gley. Most common soil hues are gley, R, YR and Y. Gradation of each hue may be expressed as 2.5, 5, 7.5 and 10. The purest hue is 5, and lower and higher grades indicate mixtures ot other hues."
Lastly, here is more technical info about soils: