This is related to the coloring process of paper tray product, mostly it is done in the pulp making process by adding dyes , then dye density changes as the pulp density changes in the following process; also most dye materials are temperature sensitive, the color would change in the drying process with the temperature. In hot pressing process, the hot press mold temperature is between 150℃～250℃,  so if you need perfect paper tray product color, adding dye in the pulp making process is not adviced.