All it takes, to look somewhat good, is the multiple-stage change of the planet surface texture, to greyish, brownish version with clearly visible burning areas. Then, once the bombardment stops and planet is recolonised, the texture will again return in several stages to its original version.
This cant be difficult to implement, the only difficult part or challenge if you wish, is to make the continuous change of the texture to look really "continuous", not abrupt and in turn comletely unrealistic.