Signs of prostate cancer depend on how far along it is. In its earliest stages, you really see or feel no symptoms. As it advances, you could have trouble uninating or see blood in the urine. A psa lab test could (but not always) be quite elevated if there is a milignancy. A digital rectal exam could (but not always) find a bump or nodule. In more advanced stages there could back pain, etc. Only males of course have a prostate.
As for colon cancer, some signs could be blood in the stool or an overall change in bowel habits. Do a google search and you'll find a ton of info.
The prostate lies along the passage of urine. It secretes a fluid with the semen during intercourse. The colon is the large bowel, which processes feces.
Knowing these things- the symptoms are pretty simple to guess. Blood in the urine may mean prostate enlargement or cancer (among other considerations), and obstruction with flow or any other problems voiding would mean evaluation of the prostate. Blood in the feces or changes in bowel habits will raise an evaluation of the bowels.