A soap without any perfume would be good, like simple soap. But I think anything would do!
It won't be a bright as it was when it was first done, as thats fresh damaged skin. But keep moisturising it well, maybe with something like oilatum or E45 and once it is fully healed, I use bio oil to keep my tattoos looking bright as possible.
But it is normal for the colour to drop in tattoos. Depending on where it is, what colours have been used and if you have tanned skin or sunburnt your skin?