Soldato
Until a light shines on you at night your bright yellow tshirt will look exactly the same as a dark black jumper. Dark black. Anything reflective will stand out.
That's exactly the point. Something more reflective is going to reflect more of the light from a car's headlight back into the eyes of the driver. Yellow is a more reflective colour than black. But yes, proper hi-vis gear is itself a lot more reflective than just a plain yellow shirt.