I'm not sure there was ever a rule saying the away team
had to wear their away kit if the colours were not the same or similar but many teams do. My own view is that I prefer teams to wear their no.1 kit away if the colours do not clash with the home team. I do wonder if the shirt sponsors sometimes stipulate the away colours should always be used. Additionally, of course, if away kits were rarely used the club wouldn't sell as many