It all depends on the game that you are after. Since this is a close range weapon (normally under 20 meters), color or camo has no bearing (unless of course you're using a silver, or neon color barrel).
Unlike target shooting, you DO want to take advantage of internal ballistics, so a 6 ft. barrel is better than a 4 ft. That being said, especially with a 0.625 "BigBore", it is going to take a lot more air to get the dart moving with sufficient velocity, and at a flat enough trajectory, to produce a clean humane kill. Therefore, you need to practice with the barrel you intend to hunt with, using the darts you intend to hunt with, shooting from the maximum distance that you think you'll hunt from. To verify you are hitting with sufficient force, you can either get a ballistics gell mold, or better (sorry to waste food), a bone in ham with skin - to emulate game penetration.
0.40 cal. - small game (squirrels, pigeons, etc.) 0.50 cal. small to medium-small game (up to racoon, pheasant, etc.) 0.625 up to medium to medium-large game (groundhog, turkey, geese). Although with a lucky shot, larger game (deer) can be downed with any caliber ... its just that the chances of doing so are slim to none.
Always best to use a spearhead dart, or a broadhead dart when hunting, to leave a good wound channel, in case of a non-lethal first shot.