no you cant get back to true pures that way.
The red is donimate, so with F1 crosses (the first offspring produced from the red x mutation cross) all will be red goldens.
It's just like breeding a black chicken to any other colored chicken, all theirs will be black.
Anyway, these will be red, but carry one copy of the mutation color gene as well. It takes 2 copies of the gene for it to be visable.
So, if you breed your F1's back to the mutation, OR to each other, you will then start getting the mutation colors again.
I dont believe the yellow was outcrosses though (maybe, not sure) but if it was, it wouldnt be a mutation, it would be a hybrid.
The fuss is, when you cross these, it doesnt strenghten eithers line, the red or the mutation. It actually dilutes them, seeing how they and their future offspring will always carry the others genes. Bad thing about it is, it's not visable, so you wont know if they have the gene without breeding them first, even then some are just splits.
and yes the big fuss is over species crossing too, it's even worse, genetically speaking than mutation crossing.