It's up to you. Your cancer is shortening your life. If you find alcohol enjoyable, and you would like to have some, I don't see a reason to deprive yourself. Just make sure your doctor knows and gives you some guidelines about how much is safe, as it could affect how your body metabolizes other medications you are taking. Drinking alcohol in moderation will not change your prognosis.
Depends. If you are on chemotherapy - no. If not and you wish to, see dr. Websters answer. Alcohol is metabolized by the liver and could effect the metabolism of other drugs you are on including chemotherapy.
Probably not. In general, if you shouldn't be doing something when you don't have cancer, you shouldn't do something when you do have cancer. I don't know of any benefical effects of alcohol. Many chemotherapy treatments put stress on the liver which will be compounded by alcohol. Also, colon cancer tends to metastasize to liver, so why risk further liver damage?