It just looks weird if one pilots wears blue with silver stripes and the other black with gold stripes.
Does it really matter? It might help identify the operators pilot if a pax walks into the lounge, the worlds military don't wear the same uniform. Not Eurpoean but our uniform at one stage was light tan pilot style shirt, brown trousers, black shoes and United States Navy leather flying jacket, no stripes.
A uniform helps, a major US airline had a crew scheduled to ferry a 707 sans pax so they rolled up sans uniform, folk in the terminal saw the non uniformed crew in the cockpit and reported to authorities that someone was trying to steal the aircraft, airline later promulgated an order that crews would always wear uniform when on duty.