It’s hard to say without someone who’s bred them right? Could be more than one allele that controls this. If it isn’t, and is a single trait double recessive cross then an F2 backcross might work.
F1 Leucistic (ll) x wild type (wt) = Ll
F2 Ll x ll = 50% ll
Then pull the F2 ll and you could be set w your new pop.