Just get a longnose gar and buy some Krylon temperature resistant marine paint.
The gar won't do well outta water for more than about 10 mins so you gotta make it quick. Make sure you tape over his gills and his nostrils on the snout. Also tape or rubber band his mouth closed. Apply the pain quickly as it will need 5 mins to dry. Painting the underside of the fish is optional.
Once dry release the fish back into the water and wait 30 mins then perform a 50% water change to remove some of the paint that make have come off in the water. Do NO MORE than 1 coat/day on your fish as multiple coats in a day cause stress the fish and offset water quality. 2-3 coats usually does the job pretty well