It's impossible to know exactly what additives are put into the gouaches.
They are formulated for painting and they don't care much about
marblers....not enough of us. The best bet for consistency is to use proper
marbling paints or to learn to adjust the other colors with dispersants so
they expand about the same amount as the weaker ones. Then when you have
it all figured out, sometimes the next tubes of colors....the same
colors....you buy are all different!
Gouache can be worked with pretty well...it's just a bit tricky at first.
Also the colors that are pale, put them down first...the following colors
will condense them.