I'm a German studying in the UK. Generally I tend to use the spelling I've seen used most in the English stuff I've read so far, both on- and offline. In the most common cases like "colour" vs "color", I often have to make a conscious choice. I usually like the British spelling more for some reason. Maybe because American spellings have a tendency to look "lazy" to me, and I'm a bit of a spelling nazi.
Then there are some words where one spelling looks closer to the one used in my native German. "Aluminium", for example, is identical to the German spelling except that German always capitalises proper nouns. Probably that's the reason that I still can't accept "aluminum" as a proper spelling. Or pronunciation, for that matter. "A-loomin-um." shudders
But since this thread is about spelling on the web, and not my private pronunciation pet peeves, I'll save that for another time.