This work modeled and compared the heat strain and discomfort differences between black and gray t-shirts. Biophysical properties were measured for 5 ensembles using a sweating thermal manikin. These results showed little differences existed across each of the ensembles, where at 0.4 ms the ranges were between 0.88 0.92 clo and the evaporative potential imclo ranged from 0.51 0.54. Spectrophotometric measures differed across the ensembles, where reflection 0.24 0.49, absorption 0.5 0.75, and transmission 0.01 0.04. Predictive modeling showed that heat strain responses were similar for all of the ensembles. While overall the wicking gray shirt had generally better and the wicking black shirt had least favorable values the differences in impositions to the human are relatively negligible. Similar to the heat strain predictions, modeling for thermal sensation discomfort showed very little difference between each of the clothing ensembles. Results support the interest of the Army for purchasing and implementing the replacement of the former gray physical fitness t-shirt with a new black physical fitness t-shirt.