After the first 3-color pour dries, you can see if you need to adjust color or darken any areas. And do another 3-color pour, using the pipette filled with watered down pigment to direct the colors into the areas you want. Let the run-off pour into your container and dispose of that (it will be a muddy brown color from mixing the 3 colors).
Let that dry and then, if you have what you want, remove the masking fluid (remember, all that grey stuff is masking fluid and all those areas will be white and hard-edged on your painting).
So...do you want a lot of hard edges? No? Then take a nice scrubby brush (lots of art supply stores have them and I think Cheap Joe's calls his Fitch Scrubbers). The brushes are just short, hard-bristled brushes. Go easy on the paper, here, or you'll scrub off a lot of the paper!
Prewet the edge you want to soften by just dropping a small drop of water along the area from the scrubby brush, wait a few seconds and then gently scrub away the color a little, working on small areas at a time - don't scrub on your paper if the area is dry because it will scratch the paper.
After you do this, it is almost impossible to go back into that area with paint - it will look bad and rough. So...make sure you want those softened edges where you want them.
And then - you are done!
The finished painting with a lot of edges softened.
Hope you try this and have fun with it. You really only need a little paper, 3 tubes of color (yellow, red, blue), some masking fluid, a pipette to direct the colors, and a tub to contain the overflow while you're pouring.