Interesting data. Your OAT was pretty low. I imagine on a 100 degree day, there could be a real problem on the gray surfaces. Definitely shows us why people use white paint, particularly in Arizona.
Some of your nighttime measurements seem to indicate the error is small, but when I was helping a guy study temperatures in electronics with an IR thermometer, he said you really needed to calibrate for the properties of the particular sort of surface you were dealing with, as they don't all radiate the same. I'll admit I don't know a lot about it.