Ok, quick easy question

I have painted my interior blue already. but i want to take all the vents off now and paint them silver. is it ok to do this? should i try and take all the paint off first and then repaint? what should i use to get the blue off? and to put the blue on i cleaned the vents with dishwashing soap, alcohol and then i put on 4 coats, 1 hour at a time.... but it still started to peal... any other ideas? or should i just put on silver dash kits. thanks