That's a good question to ask your doctor, for clarity. When the doctor tells you that you have a new cancer, it means that the cancer in your colon is not related to your first endometrial cancer, but the cancer cells originated from somewhere else. I haven't seen your reports, but I can only guess he means they took a look at the cancer cells in your colon and determined that you have colon cancer. If your endometrial cancer had spread/metastasized to the colon, it would still be called endometrial cancer.
Here is information on colon cancer from the National Cancer Institute and American Cancer Society.