I have been asked to make a rover car cake for a triple brother and sister birthday, It has to be silver grey, any suggestions how to do this without making it inedible??????

Are these being made as individual toppers on cupcakes....or as one large novelty cake? (You posted this in the "Cupcakes" forum.)

If you don't want to use silver (I'm supposing the brand you have is inedible? There are many on the market which are indeed food-safe) then you can simply make it grey and dust it liberally with edible pearl/snowflake iridescent powder/disco dust. That makes it look silver.