Their skin colour is not described, period - they may have been the same colour, they may not have been - and remember that you don't need a mixed-race marriage to produce children of different skin colours: a husband and wife who are both "middle tone" can have children of very different skin colours (case in point: the case of different-coloured twins often touted by Creationists; the parents have the same skin colour, but are middle tone - with the result that one of the twins is quite white, while the other is quite black).