Spellings

[JisthenewK]

I have just now noticed that I haven't been spelling things correctly. Sorry! I am American, but I speak to my Australian family and friends so much on Facebook. Today I posted about getting my hair dyed a "light brown colour" and some girl complained, because it is "color" in the United States. I have gotten so used to spelling things this way (favourite, mum, etc) and now I am just noticing. How did these spellings in the US form after the British spellings were already established? Why is this the only country to spell things this way? Does this make my spelling wrong?