That's a common contract negotiation protest tactic with the SPVM. Our union does something similar, but it's pretty much limited to union provided coloured slip-ons, union logo ball caps and coloured boot laces.
In Montréal it prompts the public to start asking questions which then puts pressure on the SPVM management and city hall to "get on with it". Federally, the Public Service Labour and Employment Relations Board has ruled that the employer cannot discipline member for wearing union swag on the job during contract negotiations.