[AtlUmpSteve]

Here's how I handle this with the adult rec leagues I contract with. If the league wants a uniform rule, then I tell them that their staff is to make all decisions if that one shirt is sufficiently matching the others, that my umpires are all "color blind".

The umpires will monitor unique numbers to match the lineups, and the umpire will note if an opposing team wants to protest the jersey on another team's players; but their "duty person" or "park monitor" or "field supervisor", or whatever they call their representative, must either rule on the issue, or else report to the league supervisor for his action (to declare it a forfeit, an ineligible player, whatever). We start, and continue the game until and unless the staff person declares the player ineligible and that creates a forfeit.