Well, now you’re giving information about your real-life parish. In the hypothetical parish you described in your OP, you made it sound as though shorts and flipflops had long been worn by most or all readers, ministers of Holy Communion, etc.,as standard practice.
It would be even easier and even cheaper to comply if they didn’t have to go out and buy new clothes at all. They could wear full-length pants, knee-length skirts or longer (for women), shirts or blouses with elbow-length sleeves or longer (for both sexes), and so on, which they presumably already have hanging in their closets. And colors don’t detract from the Mass. We’re Catholics, not Quakers or Calvinists.