[StuBeeDoo]

Your resin supplier should have appropriate tints to suit the resin that you are using.

Unfortunately their range is limited.  They don't offer either a silver or a grey.  My next step is to try adding a tiny amount of black to a larger amount of white, in an attempt to get somewhere near to a shade I find acceptable.

I've tried, in the past, airbrushing my homemade inserts, but I can't get the paint to adhere - even after using plastic primer.