She's not colour blind. She's simply looking at the world through half-closed eyes, like Kana puts it, paying the very minimum of attention to things. She could immediately tell something was wrong about Fujioka, but couldn't recall what it was, having removed all extra details from her mind.
Touma being so good at sewing was certainly a surprise. She must have had lots of experience doing it. Maybe because their mother is never at home and she needed to do it all by herself. But even so, she's still an elementary schooler, so there ought to be a limit to what she can accomplish.