A word or phrase that means exactly or nearly the same as another word or phrase in the same language
A word shared by two languages or dialects.
There are homonyms, homographs and homophones but none of these describes words which mean the same and are spelled differently.
'Color' and 'favorite' are the American English spellings; 'colour' and favourite' are the British English method. They are pronounced the same in that persons on either side of the Atlantic would pronounce both pairs of words the same.
Thus the pairs of words sound the same, mean the same, but are spelled differently.
This was dealt with, previously, in Stack Exchange :
What term means "words that look alike and mean alike, across two languages"?
The conclusion was either 'cognate' or that there was no technical definition.
'Spelled' and 'spelt' both mean the same, but are pronounced differently and have different spellings. They are both, currently, in use in English.
There are words within British English that sound the same, mean the same and can be spelled differently, but, usually, it is because one spelling is older, such as 'intreat' and 'entreat', the second being considered either dated or archaic. Thus one effectively drops out of use and is no longer considered the 'proper' way to spell the word.
Within English there are many words of similar, or even identical, meaning which come from different sources - Latin, Greek, French and so forth. These are called synonyms and since your example is one of different countries but with the same language, then 'synonym' would be technically correct.
Homophones sound the same, but have different meanings.
Homographs are written the same but sound differently.
There is debate, apparently, whether a homonym must be both a homophone and a homograph :
'Flower' and 'flour' are homophones; sounding the same, but with different meanings.
'Sow' to plant seeds, and 'sow', a female pig, are homographs, written the same but sounding differently and with different meanings.
'Well' meaning a waterhole and 'well' meaning healthy are spelled the same, sound the same, but have different meanings and are usually considered to be homonyms.