As a rough guide to how dark the pigmented solution should be, once you get it mixed up allow a drop or two to fall on a piece of plain white paper. It should be absolutely opaque, that is, you should not be able to see through the pigmented solution to the paper.

Pigments vary a lot in strength but if you are using tube watercolors I suspect that yoiu will need something on the order of 8-10g of pigment per liter of solution.