Is liver metastases from colon cancer a death sentence?

It could be commuted. Until recently, colon cancer metastases in the liver were invariably fatal within a few years. However, there are some new biotechnology findings that suggest this may no longer always be the case. Your oncologist would be the person to ask.
Their are survivors. With surgical resection there is medical data to support 20% of patients can be 10 year survivors in this situation. Surgery is becoming more common in this situation and additional local treatments such as radiofrequency ablation (thermal ablaion) and stereotactic radiotherapy (sbrt) are also quite promosing.
Absolutely not! Liver metastases may be treated in several ways. The chance of survival depends on resectability of the metastases from the liver. With some of the newer chemotherapies longer term remission and cure are possible. Speak with your oncologist.