Kim,
Ameraucana for what ever reason are plagued by the commercial picture of the \"Easter Egger\" that comes in multiple colors and lays pink, yellow, green, and blue eggs . . . to paraphrase a recent ad I saw.
The APA and ABA define a variety a breeding true 50% of the time . . . that is why and how blue and blue wheaten can be standard varieties.
There are eight standard varieties of Ameraucana: black, blue, white, buff, silver, brown red, wheaten, and blue wheaten.
To answer your question . . . the birds in a white x silver cross are Ameraucana in type, but they are not a standard Ameraucana variety and therefore are not an Ameraucana. The club adopted a statement several years ago (I was there) that stated firmly that mixed variety birds should be considered \"Easter Eggers\". The discussion covered all angles, but in the end landed with the strongly worded statement to help combat that perception (and reality in some instances) that as long as a bird lays a colored egg that it is an Ameraucana.
All that being said if one only had a white and a silver needed to cross them to produce more of each variety, I would. Sometimes you have to start with what you have and work from there. Last year I lost my silver bantam cock before breeding season. I made the decision to put a very nice blue wheaten cock over the silver hens. Not an ideal cross at all, but it was what I had at the time. I now have a group of F1 birds that are ready to cross back to their parents and from the F2 generation I will get a few silver and a few wheaten, which is what I want. When I sell the birds off in May I will sell them as mixed bantams and will not even indicated that they have any Ameraucana blood, because they are not Ameraucana. They are what I call \"barnyard bantams\".
I hope these ramblings made some sense! I am off to collect the eggs before the bad weather and then winter storm get here to Central Missouri! 75 degrees with a flood watch and winter storm watch . . . we should be to 20 degrees by tomorrow afternoon . . . no eggs for a few days!
Michael
PS - I wonder how/if the Leghorn, Orphington, or Wyandotte folks deal with this issue or if it ever comes up for those breeds.