[Kingstonian]

Another vote for as many shades as your budget allows.
Mine range from the very dark British tan to almost white and as others have mentioned, as the weather warms, the trousers get lighter.
But if I had to limit myself to the one colour it would be the khaki as in my opinion they are the most versatile.

I have never seen anything described as "British Tan" in Britain. I have seen plenty of Khaki though.