  Foreground galaxies that amplify the light from background quasars may also
dim that light if the galaxies contain enough dust. Extinction by dust in
lenses could hide the large number of lensed systems predicted for a flat
universe with a large value of the cosmological constant $\Lambda$. We look for
one signature of dust, namely reddening, by examining optical-infrared colors
of gravitationally lensed images of quasars. We find that the lensed systems
identified in radio and infrared searches have redder optical-IR colors than
optically selected ones. This could be due to a bias against selecting reddened
(hence extincted) quasars in the optical surveys, or due to the differences in
the intrinsic colors of optical and radio quasars. Comparison of the
radio-selected lensed and unlensed quasars shows that the lensed ones have
redder colors. We therefore conclude that at least part of the color difference
between the two lens samples is due to dust.
  From the color difference between lensed and unlensed radio quasars (and
assuming Galactic extinction law) we can reconcile a large cosmological
constant ($\Lambda=0.9$) with the number of lensed systems observed in flux
limited optical surveys. These results substantially weaken the strongest
constraint on cosmological scenarios that invoke a non-zero cosmological
constant to explain age discrepancy problems, satisfy predictions of
inflationary models of the early universe and play a role in large scale
structure formation models. They also raise the prospect of using gravitational
lenses to study the interstellar medium in high redshift galaxies.
