Another example of this tonight in the Everton v Watford game. Watford, normally yellow shirts, black shorts. Everton, blue shirts, white shorts. However, Watford were playing in all green. Why? There was no colour clash.
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Could be wrong but I believe this was addressed some years ago when supporters clubs asked the same. I think the answer was that the kit makers(Admiral or whoever etc) have a deal whereby each change strip must be worn a certain amount of times in order to generate sales.