  We present combined optical/near-IR photometry (BVIK) for six open clusters -
M35, M37, NGC 1817, NGC 2477, NGC 2420, and M67. The open clusters span an age
range from 150 Myr to 4 Gyr and have metal abundances from [Fe/H] = -0.3 to
+0.09 dex. We have utilized these data to test the robustness of theoretical
main sequences constructed by several groups as denoted by the following
designations - Padova, Baraffe, Y^2, Geneva, and Siess. The comparisons of the
models with the observations have been performed in the [Mv, (B-V)o], [Mv,
(V-I)o], and [Mv, (V-K)o] colour-magnitude diagrams as well as the
distance-independent [(V-K)o, (B-V)o] and [(V-K)o, (V-I)o] two-colour diagrams.
We conclude that none of the theoretical models reproduce the observational
data in a consistent manner over the magnitude and colour range of the
unevolved main sequence. In particular, there are significant zeropoint and
shape differences between the models and the observations. We speculate that
the crux of the problem lies in the precise mismatch between theoretical and
observational colour-temperature relations. These results underscore the
importance of pursuing the study of stellar structure and stellar modelling
with even greater intensity.
