"What will White bird X (other bird) produce?" is a question we get around here quite frequently. Unfortunately it's impossible to say - the white genes, both recessive and dominant, act as an "off switch" for other color genes. Another way to think of it is as a coat of paint. A bird might be black, or silver laced, or buff, but if you throw a white gene into the mix, you're got a layer of white paint on top of all that. Problem is, the bird is still genetically any of those colors, it's just hidden. Either knowing the parentage or test breeding is the only way to know what is hidden under that white.
Beyond that, you have the issue of whether it's dominant or recessive white that the bird is carrying, which will affect the outcomes of crossbreedings as well. Dominant white means that you need only one copy of the gene to express the color (or rather, lack thereof). Recessive white means you need two copies. In this particular case I can't find any references regarding whether White Polish is dom. or rec. white, or potentially even Silver (which is a whole other gene entirely, but less commonly seen being used to make pure white birds).
Here's a quick generalization to what kind of offspring you might get:
If your roo is dominant white:
Dom. white X Buff: White, but with a lot of red-gold leakage. Very possibly more red than white.
Dom. white X Silver laced: White with plenty of black leakage, lots of it probably closer to the tip of the feather.
Dom. white X Golden laced: Some white, but tons of gold leakage and some black.
If you roo is recessive white:
No way to tell. All the offspring will look like whatever color he is hiding X the hen's color. They will all carry white, and so can produce white offspring if mated to another white or white carrier.
