British and American spelling are different in several areas. These differences are mostly caused due to the tendency of British English to retain the spelling of words it has adopted from other languages (such as French), while American English has adapted these spelling to reflect the way the words actually sound when they are spoken.
If you are writing for British readers, you should only use British spellings and vice versa. However, for academic examinations like O Level, A Level, IELTS and ACCA, which are conducted and monitored by British Awarding bodies, it is advisable to use British English.
N.B: NEVER mix and match British and American English in your writing.