My brother and I both tested positive for colorblindness, but neither of our parents are?

X-linked. Color blindness is x-linked and you got it from your mother. I take it that you are a boy/man. Your mother is not affected because she has two x-chromosomes and only one is defective. You and your brother happened to have inherited the defective x-chromosome from your mother. Since men have only one x-chromosome inheriting a defective one manifests itself as color blindness.