Randomized phase III trial to compare the effectiveness of fluorouracil given by infusion with that of fluorouracil plus eniluracil given by mouth in treating patients who have metastatic, recurrent, or residual advanced colorectal cancer. Drugs used in chemotherapy use different ways to stop tumor cells from dividing so they stop growing or die. It is not yet known if fluorouracil is more effective with or without eniluracil for advanced colorectal cancer