I used a small amount of black oxide mixed in water ... added that to about 200 gm of soap mix. I poured the soap in the moulds then added about 100 gm of the black soap mix down the centre of each mould.
One mould I barely mixed the black into the top and the other I did quite a bit of mixing in the mould ... trying to go no deeper than half way down the mould. I did go a little deeper in places, but I'm still pleased with how they look!