From my experience you can't really overlap the wilton mists as they start to become all mixed together and look like a ugly brown or black.Spray each color on a different area and try not to overlap too much.The colors are great once you get the look you want.Also use a papertowel in the beginning to do a few test sprays.
Yes..You have to spray in layers so that the color won't puddle and drip.
Do you mean you want part blue, part pink and part red? not to use all three in the same area right?
I use papertowels to mask off the parts I don't want sprayed. I tear them into triangles and pieces to fit the design. Then I use pennies to hold them down if needed.
I would spray one color, let it dry, mask off the next area, spray second color repeat.
I have an airbrush now though so I don't have to do this anymore. : )
make sure you shake the can in between sprays to avoid it squirting or dripping out. also, i would suggest covering the area surrounding your cake with paper towels because of possible over-spray.
as for color mixing, i find if you combine two colors that blend well together you can achieve a really nice effect. for example, yellow and pink make a pretty coral - orange color. i have sprayed flowers with yellow and then lightly sprayed the petal edges with the pink. it looks really pretty. try different color mixes on the paper towels first. that way, you'll know how its going to blend on the cake.
other good combos are blue and purple, pink and orange, purple and pink.
HTH
have fun 
Oops, I went back to read and you are wanting to mix colors to get different colors? Sorry, I misunderstood. I haven't tried that myself.
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