My rivers are shades of dark blue with light blue flecks. The dark blue is mainly in the middle.
I agree with Martin: this might not be realistic but better than the alternatives. Green would blend into the surface of the table, whilst brown would end up looking like a road.
The other issue is whether the varnish will be as flexible as the latex lengths.
I have faith in this product & will varnish a single length tonight to see how well it works.