Maybe I'm confused by bill vs color sexing. I can't find where I read it, although I really thought it was in storey's but maybe not. Here is a link on it though.
https://www.backyardchickens.com/t/429552/help-with-welsh-harlequin-gold-silver-phase-genetics
Your correct on the down sexing thing Jules. You need a gold drake, and silver duck to have sex linked ducklings. But as long as their pure WH, you can sex them as day olds by the bill. Males have black, females pink.