You don't have to match color frames or similar color artwork. I think you just do what feels right for you. This is a mix of my children's art work and a few pieces I did in college.
I always look for frames if I'm in an antique or thrift store.
I came across this flower drawing my daughter did a couple years ago and I'm just hanging it up now. This is one of my favorites,
And my other daughters hand. A valentine she made in preschool.