The negatively-charged nitrogen-vacancy (NV) center in diamond is at the frontier of quantum nano-metrology and bio-sensing. Recent attention has focused on the application of high-sensitivity thermometry using the spin resonances of NV centers in nano-diamond to sub-cellular biological and biomedical research. Here, we report a comprehensive investigation of the thermal properties of the centers spin resonances and demonstrate an alternate all-optical NV thermometry technique that exploits the temperature dependence of the centers optical Debye-Waller factor.