Buffy, don't worry about it
and don't worry about the registration.
If you prefer the white sheep, get them - if one happens to have a coloured lamb, which can happen if even if you use a white tup, it's OK. If it's for the freezer it will taste the same regardless of colour. If it's good enough to breed from, a breeder of coloured sheep may be happy to buy it because it will bring fresh genes to the cooured flock.
If you prefer the coloured sheep, get them - if you use a coloured tup on coloured ewes, you won't get any white lambs
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If you decide to get a mix of white and coloured ewes, it's probably best to follow the RFBS advice to use the same coloured tup as the ewe. It just makes the paperwork easier and probably makes selling any breeding stock easier too. In any case, Dot is always able to sort out any difficulties with registration.
Please don't be put off the breed by the colour issuse - they are lovely sheep and most of the folk that have them are nice too.