Here is a quick and dirty table showing the temperatures, peak
emission wavelengths using Wien's Law and the waveband for peak
emission for various main sequence spectral types (plus L and T).
This illustrates why the U-B color for a F5-G0 star is red, because
the peak wavelength is in B-band for them, while it is in U-band for
hotter stars. (This ignores the 4000A Balmer break, which is an
additional complicating factor.)  You can see why we need to do infrared
surveys like 2MASS (http://www.ipac.caltech.edu/2mass/ ) to find L and T dwarfs.

main seq                                       
spec type       peak lambda    band   
        Temp    (Angstroms)
O5     50000K        580       EUV (extreme UV)
B0     25000K       1160       FUV (far UV)
A0     10000K       2900       NUV (near UV)
F0      8000K       3600       U or u
G0      6000K       4800       B or g
K0      5000K       5800       V or r
M0      4000K       7200       R-I or i
L       1500-2000K 14500-19000 H-K   
T      <1300K      22300       K    
note: UBVRI photometric system is Johnson-Cousins (old)
ugriz system is Sloan Digital Sky Survey standard (new)