Indeed I wrote lately that I changed black gravel by fine, light coloured sand.
The problem with my black gravel however was not the colour. The problem was that it had sharp edges on it.
When I was keeping corydoras on this substrate all their barbels worn away. Also the plants wouldn't grow on this substrate. I am not completely sure if it was a problem for the zebras.
I only observed that after I changed the course gravel by fine sand, the zebras were (supposedly) happily digging in the sand. And the plants are flourishing now!
As far as I can see, the grains of your black gravel are quite rounded, so I think that it won't be a problem. Maybe if it is very large grained it could hamper them digging a little. But I really don't know how important that is.