During dialysis why does the subclavian catheter have darker blood than in patients with a fistula?

Venous blood: A subclavian catheter has only venous, deoxygenated blood returning from the body. A fistula contains oxygenated arterial blood coming from the heart. This is the reason for the bright red blood seen in fistulas.

Fistula vs catheter: Dialysis catheters are placed in veins, and therefore use deoxygenated (dark) blood. Dialysis fistula is a connection between an artery and a vein. The blood within a fistula is oxygenated, and therefore more red.