Mn doped spinel (MgAl2O4) crystals have been grown by floating zone (FZ) techniques with various conditions of O-2 concentrations in the growth atmosphere from 0 to 75 vol%, with a starting composition of molar ratio x=MgO/(MgO+ Al2O3) from 0.3 to 1.0 and/or Mn concentrations from 1.0 to 6.0 at%. Optical absorption spectra and photoluminescence spectra were evaluated using crystals grown under various growth conditions. The color of the crystals is found to vary with their composition and the O-2 concentration in the atmosphere. Crystals grown under a 100 vol% Ar atmosphere, were a pale green color and emitted a strong green luminescence at lambda=520 nm. Colors of stoichiometric crystals, x=1.0, and Mg-poor crystals grown under an oxidizing atmosphere are yellow and red, respectively. Both optical absorption spectra and photoluminescence spectra of Mg-poor, x < 1.0 crystals grown under an oxidizing (O-2-Ar) atmosphere are found to vary greatly from those of the stoichiometric crystals.