This is not fully true. If you remember, "Gridlock" is set in the future - and he wears his brown suit. And "Daleks in Manhattan" & "Evolution of the Daleks" were set in the past - he wore his blue suit. And in other episodes, he wore his blue suit in present-day AND his brown suit in present-day. Not to mention in "Silence in the Library"/"Forest of the Dead", which is set in the future yet the Doctor switches constantly between his blue and brown suits!
- It was said by Louise Page (Costume designer) that it was David's choice, but mainly theres a pattern of Period episodes being brown and Contemporary episodes being blue. the Story Arch episodes are Brown Suit episodes