The C_cf bolometric factor against B−V (top panel), I−K (middle panel), and V−K (bottom panel). Black dots represent the C_{cf, M} median, while crosses represent C_{cf, Std}. Solid lines represent the least-square fits of Eq. (9) for each color index. The top panel omits the 125 C_{cf, Std}(B−V) data points to more clearly display the Middelkoop (1982) and Rutten (1984) fits, represented by dashed and dotted lines. The validity ranges of these previous relationships are 0.45 <B−V< 1.50 and 0.3 <B−V< 1.6, respectively, and, for comparison, are extrapolated in this figure up to the color index of M dwarfs.