I took the IELTS exam a week ago and on the way home I started to remember to have written 'color' on one answer and then it came to my mind: Do I loose marks if I write the American spelling of words on the IELTS exam? I did not see anywhere making it clear whether or not it is acceptable.
Official IELTS site states (bolding is mine):
IELTS treats all test takers with the utmost fairness and respect by actively avoiding cultural bias, and accepting all standard varieties of native-speaker English, including North American, British, Australian and New Zealand English.
As the commenters suggest below, it should be noted that your style has to be consistent. Once having chosen a particular standard, you must adhere to it.