i think that the coating is to prevent to light from dispersing straight out, u want the bulb to send the light through to sides and behind the bulb, that way it hits the crome reflector on the back of your headlights and then will project out. it makes for better light. the coating difference is mo major thing to my knowledge, i think its just that like different companies use different products, although they do the same thing they simply look different. Yellowish bulbs can mean a part of it is burnt, maybe from oil or residue that was on it then burnt when the light was turned on, this has happened to me. your best bet is to do what VTEC recommended and purchase new ones.