I'm thinking of painting my Vox amp a different colour to liven things up a bit, just wondering if anyone has attempted to do this and if so, any advice?
Yeah, don't do it. If you ever want to resell the amp it's going to be damn near impossible, and at that you will have to ask a lower price than the going rate. The best way to change the color without losing much value is a professional retolex job. But depending on the amp it will probably not even be worth it.

I have a Crate Palomino that was converted into a cab I know I was never going to resell just because no one would want it I used Krylon spray paint that supposedly adhered to vinyl/plastic, got the job done but it had this weird gloss to it that didn't look right on an amp. And it started wearing off where my amp head's rubber feet would always rest.

Gibson RD Silverburst w/ Lace Dissonant Aggressors (SOLD)
Electra Omega Prime Ceruse
Fender Franken-Jag Bass

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Line 6 M9