So I looked for alternatives and found watercolor inks. They are beautifully bright, and you can thin them down to a very light color. I use Ecoline from Royal Talens and Colorex from Pebeo. You can layer them quite nicely - but you have to wait till they are dry. I must admit that waiting is not one of my strong characteristics. But I noticed that my paintings get better when I am more patient...
I used Forest Green and Pine Green from Ecoline, and Siena from Colorex.
The only disadvantage you have with those watercolor inks is that they are not lightfast. However I do not sell originals - I use my paintings for Greeting Cards or for Art prints. The originals are safely put away in a binder.