The red/faint end of the synthetic catalogs looks pretty scary:

Displayed is the SDSS cmd and the synthetic cmd, including only objects that match between SDSS and the synthetic catalog (so basically, including only the objects in the synthetic catalog, excepting some bright objects that saturate SDSS). The red end consistently looks bad. The above is for a 1 degree radius field at (l,b) = (31,25).

Fortunately, things look at least consistent when moving to a higher galactic latitude at (l,b) = (31,85):

I've tried this for a number of patches moving down this galactic longitude; these are typical.

One thing to do might be to just throw out the red objects: cuts on g-r < .7, r-i < .45, i-z < .35, as well as cuts to throw out the brightest objects r>15.5, i>15, limit the objects to those plotted in red, which seem somewhat better.

Weirdly, the high galactic latitude plots of SDSS-synth look bimodal; not sure what is going on there.