To get consistently good results with enamel on any metal or alloy,
you need to do many tests to get the following right. Adhesion,
Matching coefficient of expansion temperature type of enamel, ie,
soft, medium, or hard, then wether you are enamelling in opaque
colours, or translucent.
Also wether your metalwork needs counter enamel to prevent warping.
So if it was me accepting this comission, I would require the
customer to agree to the design being suitable for enamelling. for
example, It might be that you will need to enamel on a seperate
piece of metal like fine gold,and set it into the white gold design,
to get the colours she wants.
As you have used and cast white gold before, you need to get the
exact composition from your gold suppliers to cross reference it with
the enamel suppliers to see if theres any incompatability. for
example, any white gold should be made only with gold, silver and
nickel, NO zinc, as enamels dont like zinc.
Finally, you need to run trials with the gold you decide to use and
the colours specified to see if it works. sometimes it will take
succesfully, then some time later the enamel will spall off due to
stress between the enamel and the metal. It seems to be fraught with
production problems. I hope you know what your doing.