  We have developed an analytical method based on forward-modeling techniques
to characterize the high-mass end of the red sequence (RS) galaxy population at
redshift $z\sim0.55$, from the DR10 BOSS CMASS spectroscopic sample, which
comprises $\sim600,000$ galaxies. The method, which follows an unbinned maximum
likelihood approach, allows the deconvolution of the intrinsic CMASS
colour-colour-magnitude distributions from photometric errors and selection
effects. This procedure requires modeling the covariance matrix for the i-band
magnitude, g-r colour and r-i colour using Stripe 82 multi-epoch data. Our
results indicate that the error-deconvolved intrinsic RS distribution is
consistent, within the photometric uncertainties, with a single point
($<0.05~{\rm{mag}}$) in the colour-colour plane at fixed magnitude, for a
narrow redshift slice. We have computed the high-mass end ($^{0.55}M_i \lesssim
-22$) of the $^{0.55}i$-band RS Luminosity Function (RS LF) in several redshift
slices within the redshift range $0.52<z<0.63$. In this narrow redshift range,
the evolution of the RS LF is consistent, within the uncertainties in the
modeling, with a passively-evolving model with $\Phi_* = (7.248 \pm 0.204)
\times10^{-4}$ Mpc$^{-3}$ mag$^{-1}$, fading at a rate of $1.5\pm0.4$ mag per
unit redshift. We report RS completeness as a function of magnitude and
redshift in the CMASS sample, which will facilitate a variety of
galaxy-evolution and clustering studies using BOSS. Our forward-modeling method
lays the foundations for future studies using other dark-energy surveys like
eBOSS or DESI, which are affected by the same type of photometric
blurring/selection effects.
