DISEASE CHARACTERISTICS: Histologically or cytologically proven unresectable, advanced or metastatic colorectal cancer Measurable disease Phase I: Any pretreatment status Phase II-N (chemotherapy naive patients): No prior chemotherapy for advanced or metastatic cancer and no prior adjuvant chemotherapy OR No prior chemotherapy for advanced or metastatic cancer and prior adjuvant chemotherapy ended more than 6 months ago Phase II-P (thymidylate synthase inhibitor pretreated patients): No thymidylate synthase inhibitor based regimen for advanced or metastatic cancer and prior adjuvant chemotherapy ended less than 6 months ago OR One thymidylate synthase inhibitor based regimen for advanced or metastatic cancer and no adjuvant thymidylate synthase inhibitor based regimen OR One thymidylate synthase inhibitor based regimen for advanced or metastatic cancer and one additional adjuvant thymidylate synthase inhibitor based regimen ended more than 6 months ago No CNS metastases