Kitchen island

Was the substrate meant to be installed in a different colour or is this common practice?

Is there any way of changing this to white now or would it need to painted?

Thanks

It can't be changed now so will have to be painted or you could laminate it but probably just as easy to paint.
Really the underside of the overhang should be stone just like the sides of the waterfalls, or at least that is how my stonemason does them. If using stone on the underside was going to end up pushing into using another slab then I'd give the client the option of either getting another slab, using standard white stone or a white melamine. Obviously the cost would vary between the 3 options. If you are going to have exposed substrate then yes, I would have used white melamine.