I used a technique that I saw somewhere (and I wish I had kept the link or written the name of the book where I found it). You splatter masking fluid at random, let it dry and paint a wash of watercolor over it. When that wash dries, you splatter more masking fluid, and repeat. Once you’ve finished you remove the masking fluid and have splatters of underlying color.
I need to practice my splattering methods, but I like the effect.