Thanks. If you want your material diffuse color to be non-white, you will need to disabling the 'Light flag' export setting. When the light flag is disabled, the model's vertex colors won't receive lighting from weathers, so for example if the weather is sunny it won't become more orange. With light flag disabled, it also removes some level of brightness from the model as shown in the picture below. The values are taken from vertexpaint modifier and show Day/Night colors.