I'm not too sure to be honest. Sometimes it'll be pretty obviously, like if Villa play West Ham, but I guess it's upto the club if they want to wear their away or third (if they have the choice).
When Hull played Newcastle earlier on in the season, the ref said that they weren't allowd to wear their full grey away kit, as it clashed too much with Newcastles, and they had to wear the grey shirts, and some white Newcastle shorts.
From the Hull website: "Hull City AFC followed Premier League procedures fully in the build up to the game in registering our second choice strip in advance of this fixture.
It was only when the team arrived at the ground on Saturday that the match referee, as is his right, decided that in his opinion our flint coloured away kit was in fact too similar to Newcastle United's home kit.
With Newcastle United's co-operation, we were able to wear their second choice white shorts and white socks to enable the match officials to distinguish between the two teams."
I guess teams make the choice based on their knowledge of the home side, but don't always get it right
