From color index to luminosity Given two colorindices LX and LY, such that the color X-Y is given by

X-Y = -2.5 log(LX/LY)


is there a way to infer the luminosity in X (or Y)? I'm think not because any information on units has fallen out through this definition, but I could be wrong. I'm just not sure how to compute the luminosity straightforwardly otherwise, as this color index is the only output (along with the distance modulus) from an interpolation scheme of stellar templates.

Help would be much appreciated.
You can never tell the luminosity of a star from knowledge of its color X-Y alone, that only tells you something about the surface temperature.  The problem is not solved by knowing the distance modulus, that only tells you how far the star is, but not how big it is.  If you want luminosity from those things, you need to know how bright the star looks, not just its color and distance.  Or you could look at the width of the spectral lines to infer luminosity class, but that brings in line resolution.  But if you know the distance modulus and Lx and Ly, not just Lx/Ly, then you can get the luminosity because you know its color, how bright it looks, and its distance.