Saw someone reposting this on twitter.
I've been making Larp weapons for 20 years so I've seen this a fair bit. Latex is pretty hard to damage with most paints. Anything with copper in though will cause latex to decompose, rate dependent on how much copper is present. Really solvent heavy paints might damage latex a bit but I've never seen it melt. The biggest problem you'll have is getting anything colour to stick to it for any length of time.
When latex is fresh, new coats of latex stick together very well but over time it cures and the layers of latex or paint won't stick together anymore. Spray paint might stay on for a while but typically wears off fairly quickly.
If you wanted to try though, I'd suggest washing the weapons with detergent and warm water to remove any grease or dirt. Then mix your own paint from copydex, or similar latex based glue, and acrylic paint. Build the colour up with multiple thin layers. That might work.
However I think you'd be wasting your time. If this was a weapon brought to me I'd strip the original latex off and re-latex it from virgin foam, that would be the best bet for a long-lasting recolouring job.
I talk about about sealing and painting weapons here:
Stripping the latex is easier on weapons that have been used, harder on new weapons but with a sharp blade or fine-scissors you can work around it and pull the old latex off in sheets, bit like skinning something.
Hope that helps.
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