Although with under shorts, if it is still blue under blue shorts for instance, I don't really care what shade of blue as long as it is not glaringly different. Else off they come.
I dont ever find I have contrary colours (i.e. Blue vs Red, Red wearing Blue undershorts) its just usually black tights, or black tape when neither team wear any black.
Ill take the first comment on board too.
So precheck, let them know they cant start with them.
Players lined up for kick off, check the tape/offending article again, still there, ask them to leave the FOP to get it sorted - commence kick off.
They ask to come back on - BUT havent sorted the article out (for whatever reason, probably "We cant find any other tape ref") then its a caution and they're told they cant re-enter the FOP.
OR
They just re-enter the FOP without telling me, which is a caution by itself.
Seems straight forward.
I bet this is the quickest way to annoy people, I mean, we havent even kicked off - and im hated already!
Had similar last week with a lad wearing a rubber wristband with poppies on it. Told me he wasnt taking it off and "I cant make him", so I told him to leave the FOP...
...So he took it off.