  We present Stellar Locus Regression (SLR), a method of directly adjusting the
instrumental broadband optical colors of stars to bring them into accord with a
universal stellar color-color locus, producing accurately calibrated colors for
both stars and galaxies. This is achieved without first establishing individual
zeropoints for each passband, and can be performed in real-time at the
telescope. We demonstrate how SLR naturally makes one wholesale correction for
differences in instrumental response, for atmospheric transparency, for
atmospheric extinction, and for Galactic extinction. We perform an example SLR
treatment of SDSS data over a wide range of Galactic dust values and
independently recover the direction and magnitude of the canonical Galactic
reddening vector with 14--18 mmag RMS uncertainties. We then isolate the effect
of atmospheric extinction, showing that SLR accounts for this and returns
precise colors over a wide of airmass, with 5--14 mmag RMS residuals. We
demonstrate that SLR-corrected colors are sufficiently accurate to allow
photometric redshift estimates for galaxy clusters (using red sequence
galaxies) with an uncertainty sigma_z/(1+z) = 0.6% per cluster for redshifts
0.09<z<0.25. Finally, we identify our objects in the 2MASS all-sky catalog, and
produce i-band zeropoints typically accurate to 18 mmag using only SLR. We
offer open-source access to our IDL routines, validated and verified for the
implementation of this technique, at
http://stellar-locus-regression.googlecode.com
