Welcome to e health forum.
It is possible that you may have endured pelvic nerve injury, either as a result of the colon cancer, or due to effect of the radiotherapy. Such permanent damage can cause erectile dysfunction.
In case, you are not able to have early morning erections, then it will be a sign of confirmed erectile dysfunction.
In such a case, you will need to consult a urologist, for proper evaluation. In case medications like Viagra or Cialis are not helping, surgical options including penile implants might be necessary to have a good sexual life.
I hope this helps.