Quickest is to use paint remover. I forgot the chemical name. Just go to a hardware store.
Be very careful when using it. Although it comes in gel-like form, it is very volatile and very corrosive. Wear gloves, eye protection and long sleeves shirt and use in a well-ventilated area. I've had some on my hands and I can tell you that it burns like hell.
Also, take note: DO NOT open the can with your face right over it. The lid could just pop right into your face. I happened to me.
When applied, DO NOT leave it on the racket for too long. Max it off between 3-4 hours otherwise you could inadvertently destroy some of the resin that holds the graphite fibers. (No that bad though). Then wash off under running water, using steel wool to lightly rub off the paint. Reapply if need be.
You can then slowly sand off the reluctant paint later, just be careful not too over-apply pressure or you'll have a part of the racket "eaten up".
Enjoy!