@haz, first welcome to CCF. Second, best is to add a couple pics of your coin to this thread, but in their absence I note that most often when someone asks this question here the answer is environmental staining rather than a mint error.
"If you climb a good tree, you get a push." -----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed." -----King Adz