Jaundice is normally caused by two things: blockage of the excretion of bilirubin or overproduction of bilirubin.
Any type of liver (inflammation, liver failure) or gallbladder disease (i.e infection, biliary duct obstruction) can cause jaundice.
It is possible that surgery can cause the jaundice, but this would be less likely since the surgical procedure to remove the sigmoid colon would be away from the liver.
I would suggest liver function tests and an abdominal ultrasound as initial tests to evaluate the liver/gallbladder.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
It could be a drug reaction from either his anesthetic agents, or other meds he's getting, or he could have developed postop pancreatitis or cholecystitis (inflamation of pancreas or gallbladder): those are high on the list of possibilities for postop jaundice. There are other more rare situations, such as clotting of the vein to the liver, abscess under the liver, etc. I'd assume the surgeon is aggresively investigating these possibilities.