We have completed a grid of spherically symmetric AGB star atmospheres using the state of the art spectral synthesis code PHOENIX. Models are constructed for stars with masses of 1 M⊙ and 1.5 M⊙, spanning the range 10 to 3300 L⊙ in luminosity and 2500 to 5200 K in effective temperature. We find that grains of Al2O3 and CaTiO3 among other species form in atmospheres cooler than Teff = 3000 K. In the coolest models the grains cause a weakening of the TiO absorption features in the red and near infrared of up to 30% through both a depression of the continuum and a depletion of the TiO number abundance. We use spectrophotometric observations from a number of catalogs to determine effective temperature – spectral class and effective temperature – color relationships. We also compare synthetic colors calculated from our models with observations of M giants on Wing's 8-color narrow-band system of classification photometry.