I am on follow up chemo for colorectal cancer, also...

There are a number of reasons why urine might be darker during treatment for colorectal cancer. The color of urine can change based on the medications a patient is taking; See also:
Exactly which chemotherapy drugs are cardiotoxic?

See all 1939 questions about Cancer chemotherapy agents and medications for nausea and thrush could be responsible. Dark urine is also an indication of dehydration.

If colon cancer is affecting the liver, this can cause jaundice, and tea-colored urine is an early sign of jaundice and liver disfunction. This sign can occur even before jaundice is apparent through discoloration of the skin and eyes. There is a simple test that can be performed at the doctor's office to analyze urine for bacteria, medications, and other problems. Call your doctor to ask for a urinalysis to find out what's causing this problem.