RATIONALE: Drugs used in chemotherapy, such as leucovorin, fluorouracil, capecitabine, and oxaliplatin, use different ways to stop tumor cells from dividing so they stop growing or die. Combining more than one drug may kill more tumor cells. It is not yet known whether leucovorin and fluorouracil with or without oxaliplatin is more effective than capecitabine with or without oxaliplatin in treating patients who have metastatic colorectal cancer.