Somethimes i have dark green or black stool for a day or two than it changes back to the normal colour.the dark stool appears once in many week and it's only temporary. shall i be worried about this? do serious conditions cause permanent black stool?

Black stool is concerning since this could be due to blood in your stools. The green stool is most likely due to something you are eating. You may want to have your doctor test your stool for microscopic blood to be certain you are not having blood in your stools, especially when the stools are black. If it is blood, you should see a GI doctor. Good luck to you!