Complementary colors are colors that are opposite each other on the color wheel. Yellow and purple. Red and green. Orange and blue....
Complementary colors create high-energy designs, and this can be particularly useful if you want to make something very simple that really pops...like my card above. The sun, a single word, and POP! All because of the complementary colors.
I don't recommend dressing hyper five-year-old boys in complementary colors. Choose a soothing monochromatic or analogous color scheme in muted tones. Because, you know, kids get color theory, right?
You're not buying it. I can tell.
By the way, analogous color schemes are made of three colors side-by-side on the color wheel (a primary or secondary color plus the colors on either side). They create lovely harmony and depending on the tones and color saturation, they can be incredibly soothing or nice and peppy, but they lack the contrast and high energy of complementary schemes.
I'm no expert on color theory, but it's worth reading around on the interwebs for the basics. Color Matters and TigerColor are two good places to start. I refer to my color wheel frequently and consider it to be an indispensable tool for paper crafting. Mine rests in easy reach on my cork board over my craft desk. They are inexpensive and readily available in the art sections of Michael's, Hobby Lobby, or JoAnn's.
In the words of Woody the Cowboy doll...if you don't have one, get one!
Or not. It's really up to you.
stamps: Papertrey Hello There
ink: Hero Arts orange soda, Memento Bahama blue
accessories: circle punch, dimensional