This is one the the laws have left to comp rules. Most comp rules cover kit requirements extensively but if they don't, for me, just like with the laws of the the game we use common sense, what football in general and that specific game expects.
For case of OP, comps like FA cup start at rounds in/close to grassroots and then all the way up to rounds that usually only top tier clubs are involved. If comp rules don't cover it, what I would do depends on the 'prestige' of the comp as well as teams involved. If both teams are grassroots or close to it, I won't worry about socks and shorts (as long as no clash), and happy with any shade of the shirt the rest of the team is wearing. Further into the rounds, I won't be as flexible.