  We present quasi-simultaneous multi-colour optical/near-IR photometry for 157
radio selected quasars, forming an unbiassed sub-sample of the Parkes
Flat-Spectrum Sample. Data are also presented for 12 optically selected QSOs,
drawn from the Large Bright QSO Survey.
  The spectral energy distributions of the radio- and optically-selected
sources are quite different. The optically selected QSOs are all very similar:
they have blue spectral energy distributions curving downwards at shorter
wavelengths. Roughly 90% of the radio-selected quasars have roughly power-law
spectral energy distributions, with slopes ranging from Fnu=nu^0 to Fnu=nu^-2.
The remaining 10% have spectral energy distributions showing sharp peaks: these
are radio galaxies and highly reddened quasars.
  Four radio sources were not detected down to magnitude limits of H ~ 19.6.
These are probably high redshift (z >3) galaxies or quasars.
  We show that the colours of our red quasars lie close to the stellar locus in
the optical: they will be hard to identify in surveys such as the Sloan Digital
Sky Survey. If near-IR photometry is added, however, the red power-law sources
can be clearly separated from the stellar locus: IR surveys such as 2MASS
should be capable of finding these sources on the basis of their excess flux in
the K-band.
