You can add a referee kit to a league, to a cup, and also to a specific match. Again, the tool needed for this is the "Revolution Mod". If you look at the referees rx3 files, these are a set that consists of teamID values of 6004, 6005, 6006, 6007, 6008 and 6009. The kit type of referee kits is always 5. (By default, this kits are usually 6004 black, 6005 yellow,6006 blue, 6007 red, 6008 grey, 6009 green).
Now, if I wanna put the four Brasileirao (leagueID 7) referee kits -black,red,blue & yellow-, I will create rx3 files with CM (remember to put the values of back name, and numbers in 0) , and they'll be named as: kit_6004_5_7.rx3 for the black, kit_6005_5_7.rx3 for yellow and so on. Then you can assign a collar, let's say collar is 0, and league ID 7. Put assignTournamentRefereeKitCollar(7,0) in Game>data>fifarna>lua>assignments>General file.
And again, is all explained in RM instruction manual. Cheers.