Is loss of appetite a symptom of colon or lung cancer? it is my only symptom and i have severe anxiety and depression?

Your anxiety and depression are more likely explanations for your symptoms. If you do not have cough or blood in stool, cancer is not likely. It is not feasible to rule out cancer without physical examination, imaging studies and may be a colonoscopy.

Loss of appetite is a symptom commonly seen in a variety of medical conditions, including cancers, depression and severe anxiety. Talking to your treating doc would give you the best start in having your symptom properly addressed.