The Buffs used in the original cross were the females, correct?
So with chickens the males have two "Z" chromosomes (ZZ) and females have 1 "Z" and 1 "W" chromosomes (ZW). The silver/gold base is sex-linked, meaning it only passes on the "Z" chromosome.
Here's the genetics for silver/gold:
Males-
S/S = silver
S/s = golden (silvery yellow)
s/s = gold
Females-
S/- = silver
s/- = gold
If your original cross was Lavender x Buff (always write as sire x dam, never dam x sire) the genetics would look like this:
Lavender (S/S lav/lav) x Buff (s/- LAV/LAV)
Male Offspring-
100% Golden-based Black Split (S/s LAV/lav)
Female Offspring-
100% Silver-based Black Split (S/- LAV/lav)
The golden color that your pullets have is known as "leakage". It's not because they are gold-based, it's basically "holes" in the black pigmentation where the red pigmentation comes through.
Crossing the male and female offspring of this cross will result in
Males-
Black S/S LAV/LAV
Black Split S/S LAV/lav
Lavender S/S lav/lav
Golden S/s LAV/LAV
Black Split S/s LAV/lav
Isabel (sort of) S/s lav/lav
Females-
Black S/- LAV/LAV
Black Split S/- LAV/lav
Lavender S/- lav/lav
Gold s/- LAV/LAV
Black Split s/- LAV/lav
Isabel s/- lav/lav
The next step to fix the Isabel coloration would be to cross both of the Isabel and then cross the best resulting Isabel male over his mother. All of the offspring from that final cross will breed true for Isabel.
As a slight correction to my previous post, you could get Isabel fastest by crossing a Gold Laced or Buff male over a Lavender female and then crossing the offspring (about 1/100).