Bianca's idea is good but seems to have a fundamental flaw: No one is going to stock a 720 range of tights, especially at £20-odd quid, and hence people will still be unable to find their match even if she is producing them.
And of course all it takes is a bigger producer to undercut her and steal her spot on the shelves and she's completely screwed.
It'd be a better online business, but then you'd probably get a very low order volume.
Mark's business idea is a lot more proven, a lot less risky and very easy to set up.