Shades of white gold alloys

Hi, in the past (2012) I have been able to get different white gold alloys. I have one customer who wants a wedding ring with a slightly yellow shade, I actually have one myself. The other customer wants a rhodium plate type colour. I understand there is an actual grading of Premium, Standard etc. and that Premium 9ct white will have 37.5% gold and 62% silver. Unplated white gold I have seen so far always goes a dull grey, my straw white one looks better to me, but it seems it can't be sourced now?

Liz,
Having just done an 18ct white ring commission I understand your difficulties as getting technical, info on white colours seems to be as difficult as getting blood out of a stone.
I'm hoping someone will wade in with a better answer than me but my understanding is as follows:
White gold colours can be graded as
Premium (1) - good white and not requiring rhodium plating
Standard (2) - rhodium plating is optional
Off white (3) - rhodium plating is required.

I dont know if this is yet an official grading system but Cooksons refer to it in the technical section of their 2014/15 catalogue (pg44) but they they then refer to terms such as ' strong white' and ' straw white' in their technical tables on pg 42
Mark Grimwade's book ' introduction to precious metals' also has useful info.

With regards to the gold/silver (au/ag) percentages, I would be surprised if anyone produces a straight au/ag alloy as I suspect they will usually include other metals such as zinc or copper. Again, in the Cooksons 9ct range they indicate around 55% ag but dont state the other metals.

Palladium is also used as a primary whitener but seems to more prevalent in 18ct alloys.
Betts metals were unable to give me any technical info.

If you are registered with the London Assay Office you may be able to get further info from their library section which would be my next port of call in the future.
Do post again if you find put any more as I for one would be interested.
good luck
Tim