Use scotchbrite to scuff up the paint on the bottle. Your not trying to take the paint off the bottle just knock all of the sheen off the paint. Take some paint thinner or degreaser and a white rag and wipe the bottle. Keep wiping it until the rags come off clean.
You can probably do a pretty good job with a rattle can and I wouldn't worry about the paint falling off because the bottle gets so cold. If you want a better finish I would use Acrylic Enamel with a Catalyst and a HVLP paint gun and paint it to match the car.
Also if you want to get creative, Walmart sells house of colors paint, its a 3 stage paint, so if you do like the above, and buy a scotchbrite and take the clear off, and whipe it down really well, then you could do that and make it look nearly professional, Im going to try this this weekend.
Honestly the bottle clamps will scrape off just about anything you put on ther. I did it, and just had to be very careful when putting the bottle back in the clamps. I used high temp engine spraypaint from autozone or something.
line the clamps with cloth or double sided sticky tape and dont peal off one side, it should cushion it enough to not have metal on paint. I will be painting mine when it warms up, and take a few pics of the setup as well.
Sup Ryan, its been awhile, we need to go party like the old times.... anyways I'm spraying my V8 lexus motor... when I get it all lodged in the back of this MR2. Planning on a 100 shot wet when I get her on the road...