http://xxx.soton.ac.uk/abs/astro-ph/0010052 carries a preprint carrying colour - colour diagrams between SDSS derived optical indices and 2MASS indices. Though the matter seems to be somewhat problematic, useful information/hints could be gleaned here as to whether very red 2MASS stars are liable to be certain classes of variables, and whether certain types of variables are liable to have flat or steep IR colours, &c, _or not_. John