I believe that I read on Metzers page that the second generation will also be good layers, though not necessarily as reliable. The colours will be different though. With the golden 300 the blackish ones are always male and the brownish ones are always female. This will NOT be the case in the second generation. I have no experience with the white layers.
I have 3 golden 300 that are 20 weeks old. 1 M 2 F. They have not started laying yet ;-( but I do plan to incubate eggs next spring to see what happens. Let me know if you find out before me ;-)