I played at Warhammer World against a guy who had a Night Lords painted army who was using the Blood Angels Codex. When the event organiser came over it went a bit like this:
he saw his force and said "so your Night Lords, Chaos Space Marines?" looks at note pad to mark this off
Opponent: "No, I'm playing Blood Angel codex..."
Organiser: Looks confused "So why are you playing that codex?
Opponent: "Well, because I felt..."
Organiser: *under breath* "It was a better codex"
I giggled. Later on I found out the guy had a really low painting score because they weren't painted Blood Angels but other than that they had no problems with it. I did hear last weekend they pulled a mate of mine up on his flying bases. He'd sprayed one unit's bases white and one unit black to differentiate them but having applied paint they then weren't classed as fully painted so he had to remove them! Seems OTT to me, not a fan of that arbitrary ruling about what constitutes fully painted. you may as well have people getting points knocked off for using the stock green Necron rods or unpainted canopies of Storm Ravens and Vendettas, I digress...