The Complementary Pop of Color
To review, a Complementary Color Palette contains colors that are opposite one another on the color wheel. (You can learn more about complementary palettes here.)
Colors that are opposite one another on the color wheel are as different as you can get. They share no colors in common, and because of that they can really make each other stand out. Pink will look MOST pink when next to green. Blue will look most blue when paired with orange. So choosing a complementary color for your pop will make that pop really...pop!
SO to insert a Complementary Pop, simply start with a monochromatic or an analogous palette (as you can see below, I started with blues and aquas, and to keep it interesting, I varied the values of those blues and aquas, including lights and darks.)
Then I chose a color opposite blue and aqua on the color wheel -- orange! Don't get too hung up on making sure your pop is exactly opposite on the color wheel, just jump across the color wheel and pick a color you think looks good. I like to distribute the pops relatively evenly across the quilt.
The lovely thing about this trick is that you now automatically have both warm and cool colors in your quilt, which will make it look balanced and dynamic.