Many organo- sulfur-nitrogen systems are thermochromic, although yellow and orange on heating are more common. 310 degrees and brown suggests partial decomposition giving charring.

Thanks Paul. Actually I heated 2-mercaptobenimidazole(2-MBI) along with calcium oxide at 310 degrees. 310 degrees is melting point of 2-MBI. The colour of calcium oxide is white. So the final product after cooling to room temperature was pale brown in colour. So is that thermochromic materials retain the same colour on cooling. Please explain

Not usually, on cooling the original colour is taken up, also as I mentioned before yellow and orange colours are more usual in sulfur-nitrogen ring systems on heating. What is possible is that once mercaptoimidazole melts, tautomerism takes place with one of the H atoms moving from N to S, an S - H is acidic enough to be deprotonated by CaO, and the resulting anion with a heterocyclic ring system with pi bonds is likely to have a brown colouration. Hope this helps.