Comparison of WISE absolute magnitudes in w1 (3.4 μm) band with V − K_s color (left panel). As can be seen, the two proxies for stellar temperature do not match well. This is not because of the K_s magnitude (as can be seen in the middle panel), but because of the magnitude in the visual band (right panel). An empirical determination of M-dwarf mid-infrared colors as a function of the V − K_s color index can be found in the appendix.