well, since you've had 2 specific answers, I'll try to answer the general question.
(These "rules" are not mine. I stole them from the television program What NOT to Wear
. If you're a fan, that's why these rules sound so familiar. I LOVE THAT SHOW!!!)
It's pretty simple, really, how to mix/match and make outfits. The same general rules apply for children and adults. Here are the rules.
1) There are neutrals colors that go with everything: black, grey, white, brown, tan, beige, ivory, etc. Just pick a lovely, bright color - red, green, yellow, orange, etc and pair it with any combination of neutrals and you've got an outfit. (All neutrals "go" with all other neutrals.)
2) There are pseudo-neutrals, colors that can act like a neutral: navy, khaki, olive, denim. You would chose one psuedo-neutral, any combo of true neutrals, and your "pop" color to make an outfit.
3) There are also subtle patterns that from a distance look like a solid and act as a neutral (or pseudo-neutrals) Harringbone, pinstripe, tweed, etc. You would choose one subtle pattern, any combo of true neutrals, and your "pop" color to make an outfit. I have seen very sophisticated and elegant combinations of subtle patterns work together - but it takes some skill to pull it together effectively.
4) If you want to feature two "pop" colors, say for example hot pink and hot orange, make sure at least one piece of the outfit contains both colors. Then contain that really loud "pop" by limiting it to just one part of the body - say a hot pink tank underneath a hot orange blouse with a pink and orange necklace or scarf, and make SURE that the rest of the outfit is neutral. Plain neutral, not pseudo or sublte.
This rule also applies when combining two patterns. Make sure they feature the same colors and that the two patterns are of equal visual impact. (a striped skirt with a polka-dot top, for example. It can be professional - as in a navy pinstripe pencil skirt with a navy and white tiny polka dot blouse. Or it can be casual, as in our hotpink/orange example - alternating stripes with a white background on an A-line or gathered skirt and pink/orange polkas on a white background on a blouse. The width of the stripe - say 1", should be the size of the dot, again 1".)
When in doubt, wear your "pop" on your feet! A red shoe, for example with denim/jeans and a solid color top.
5) Last rule. If you really want to create an elegant and sophisticated look, keep in mind the following: color, texture, FIT, and sheen (such as satin, gold, silver, etc) Make sure your clothing FITS your body like it was made for it!! And make sure each outfit contains at least one element of color, texture, and sheen. Now you've got a complete "look."
I had fun answering the question and I hope others benefit. Wish I had their $5000, so that I could actually accomplish this in my own life, but OH WELL... I had fun helping others.