This problem occurs on full size white painted aircraft, and is caused by the turbine oil. No way of stopping the discolouration as far as I know
I have just switched to Mobil DTE light turbine oil (approved by Wren for my engine) & it is almost clear wheras the Mobil Jet Oil 2 I was using had the yellow colour. Does the jet oil have a dye for identification or is it the nasty chemicals that turbine oils have but the DTE light supposedly does not have?
Maybe a change of oil will stop the problem occuring in the first place.
BTW, the DTE light is 1/5 the price of turbine oil in OZ. - John.