Well being it that it is a plastic insert, I would think some standard plastic model paint would work. As for the heat, I don't think there is that much heat coming off the radiator. Besides, while the car is moving, the heat is pushed back by the wind. When the car is not moving and the engine is really hot, the fan will be on.

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1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member