Long‐term monitoring of 1E 1740.7‐2942 and GRS 1758‐258 has revealed behavior very different from the “prototype” black‐hole candidate Cyg X‐1. In these two sources, as we have previously reported, the softest spectra are seen while the luminosity is dropping, rather than when it is highest, as in Cyg X‐1. The difference may be related to the accretion geometry (wind‐fed vs. disk‐fed) via the long viscous time scale for the propagation of changes through a large accretion disk.We now add an additional piece of information to the analysis: the total rms fast variability in each observation. We find that this parameter correlates better with the spectral index than either the luminosity or its derivative. The correlation has no observable lag and is particularly striking in GRS 1758‐258. © 2004 American Institute of Physics