Sometimes comes after clothes. "I wear dark clothes sometimes." You can also put it before, "Sometimes I wear dark clothes." Putting sometimes in the middle of the sentence would require commas and change the meaning of the sentence.
Not necessarily. You can say, "I sometimes wear dark clothes," without any commas or change of meaning. I just want to point out, however, that this Duolingo sentence is about colors (kleuren), not clothes (kleren).