the colored paper might do it...did you add white watercolor to any of the other colors though? It just looks so much like gouache, yet it's watercolor (and that is an incredible thing. I love gouache, but I hate how expensive it is)
White watercolor is only in ghostly people and medusas, so you're right, it's a nearly gouache, maybe. I'm not good in identification of technique I use some old, cheap watercolors, and sometimes their non-quality make some random effects.