Not everyone knows that there is British and American ways of spelling words, and they are different. The question about the acceptable spelling came up a lot lately and I found out that many students are unsure how to spell words in the IELTS Listening, Reading and Writing tests – the British or the American way.
First, for those who have no idea what I’m talking about, let me explain: if you ask someone from USA and someone from the UK to spell a word such as color, the American will spell “color” and the person from UK will spell “colour”. There are many other examples:
favorite (USA) – favourite (UK)
honor (USA) – honour (UK)
memorize (USA) – memorise (UK)
check (USA) – cheque (UK)

There is no point in listing all of the differences here, you can see the whole list on this page.
As to the question, what is the right way to spell words in IELTS, American or British, the answer is both. You can use either form and it will be accepted, no penalties.