Why thermoplastic road marking paint becomes dark?

The reasons why thermoplastic road marking paint becomes dark:
1.   Heating temperature is too high, the coating is burnt. The normal heating termperature is 180-210℃.
2.   The mixing is not well-distributed, which make part burnt and dark.
3.   The plug is broken
4.   There is the paint which is left in preheater and the paint is not cleaned when they do the marking in the second day.