Accurate color for dye sublimation starts with proper file preparation, correct printing and most importantly the time, pressure and temperature that the heat press uses to complete the dye sublimation process.
There are several methods of measuring accurate temperature of your heat press, rather it be a simple clamshell press or larger oil filled rotary heat transfer press for industrial use.
Quite common is the little IR thermal measurement gun and it is a good, quick test, but difficult to get an accurate reading and fluctuates based on what surface and where it is pointed. The expensive digital readers with temperature sensing probes don’t do much better, but meet a need that is used in manufacturing and you can even get a probe that will fit most VOM units in the K slot.
Our go to temp test is the little heat strips that come in a packet and you simply take one off, place it on paper and run it though the press like you would any production. That way, you get the time, temperature and pressure of actual operation and the temp strips are the most accurate.
Any dye sub and heat press operation should at the very least have these available to properly verify that you are getting the temperature out of your heat press consistently thus eliminating any variable that might get you into trouble with production errors.