Please do look at the graphs shown at the link
I mentioned in my earlier posting. You will see
that there is quite a wide spread in the spectral
types of stars with identical Tycho colors. That's
partly due to errors in the Tycho magnitudes, and
partly due to the intrinsic spread in properties
of stars.

If all you want is a bunch of I-band magnitudes
for Tycho stars, you can grab them from a datafile
the TASS group has been using for some years
now. I picked a subset of stars from Tycho which
have reasonably accurate photometry and no
close Tycho companions. See

http://spiff.rit.edu/tass/pipeline/p.../pipeline.html

and read down a few pages to the point where the
catalogs ("Big Version" and "Small Version") are
described. I provide estimate I-band magnitudes
along with estimate Johnson-Cousins B and V,
based on the Tycho Bt and Vt.

Michael Richmond