Do you tup them as hoggs? I don't til they're shearlings, my hoggs aren't fully grown.
Anyway: animals that are white, from the white tup, may well also have a different gene from their mums. Ie if mum was moorit, they'll have a moorit gene from mum, if she was a grey katmoget they'll have both a black gene and a katmoget gene from mum.
So a moorit tup, would give the lamb a 'self-colour' and a 'moorit' gene, and if the white hoggs give their lamb the gene they got from their mum, you'll get different colours, if they give their lambs the white gene they got from their dad, you'll get white lambs again. It's a 50:50 chance, so I suppose on average half your lambs will be white and half coloured, though of course it never works out perfectly to statistics!