buy a cheap probably used plastic squirtgun and brass and copper spraypaint, and some craft paint to make copper-green.
Step 2: Disassemble the gun completely
you will need one of those tiny thin screwdrivers that are used for electronics since the screws will be in thin slots
In this case, i removed the tubing and the pump action handle since it was broken anyway.
Step 3: Spraypaint the parts different colours
once the parts are taken apart, spray paint them with the least amount of paint you can, and hang the tank parts by string from a clothesline or branch or something to dry, lay the flat parts on a box or newspaper.
Step 4: Reassemble the gun and smoosh green paint in the corners
Once the gun has dried for an hour or so, the patina is just smudged green craft paint in the corners while fairly wet, rubbing the exposed areas with your fingers to clean it off. spritz various areas by running fingers through the brush to make spots. Anywhere there is a corner put in some patina.
Step 5: Different views
you can decorate it then as you want. but that is personal taste.