Is Millie the one with the long ears or is that Molly? Are you going to breed from both? Really need to stagger the breeding (one this year, one next) so you have milk pretty much contast.
A lot of goats which are x breed have sanaan in them, which are the white ones. So, that could be the white, coming through when the coats change from summer to winter.
A lady wanted to buy Poppy and Primrose my two white kids - she thought they were pure bred Sanaans but they are not. Mum was a sanaan, but dad was a big British Alpine - black with a white stripy face and white legs. I was very honest and told her the truth (but she still wanted them!) So, if I breed from them, they could have kids the colour of their dad, or patched black and white (have had one that colour before now!)