White does not dilute the pigment. It turns off its expression, but depending on the specific white (there are two separate genes, each of which have several alleles) genes & alleles resulting from the cross, you might end up with chicks who are all white or all buff; the more likely scenario is that you'll end up with a bunch of genes that were hidden making their appearance known--and what those genes may be is completely unpredictable--about the equivalent of saying, if I breed my buff hen what colour will the chicks be (notice the lack of mention of a rooster).
If you want to dilute the buff, you need to add a gene that dilutes red pigment: lav, di, cb or ig.
Obviously I know what the first one is.... what are the other three?
Lavender, dilute, champaign blonde, & inhibitor of gold. lav & ig are recessive; Cb & Di are dominant (Di is incompletely dominant), so I should have capitalized their symbols. Lav also dilutes black pigment, and Di has some affect on black, but not as much as on red.