Paintball Boxes
These paint boxes are the ammunition for Digital Paint. The balls are loaded into the hopper from the boxes. You begin the game with 10 paintballs and must pick up boxes in order to be able to shoot more. The boxes come in sizes of 25, 50, 100, 150, and 200 Balls. You must grab a Hopper in order to carry more then 40 paintballs. It is recommended that you only grab as much paint as you need so that your teammates will also have ammunition.
Barrel Upgrades
Barrel upgrades allow your gun to shoot with a higher velocity. Which in turn makes the paint fly further. The distance your paint flies is determined by velocity and angle of the shot. Gravity effects the paintballs and they fall just like any other projectile. Thus a barrel which increases the velocity of your paint is very important. Their are 3 variations of Barrels. 

Steel Barrel multiplies speed * 1.15
Chrome Barrel multiplies speed * 1.10
Brass Barrel multiplies speed * 1.05

Co2
Co2 is a gas of course, which provides the pressure necessary to shoot your paintballs. All guns in the game run on CO2 tanks. They come in four different sizes. 12 Gram, 7 oz, 12, oz, and 20oz.Obviously the bigger bottle the better.
Paint Hoppers
The hopper is yet another important part of your Paintball gear. A hopper is the container that attaches to your gun to hold the paint. In Digital Paint you begin with a 40 round hopper and thus can only carry 40 paintballs. If you find a hopper upgrade you can then hold more ammo. Hoppers in the game come in 3 variations. 100, 200, 300. Of course you would want the largest hopper possible. If you want to drop a hopper you can use the bind drop_hopper to do so, and then pick up a larger one. However you will lose whatever balls were in that hopper, except for the 40 that fit in your default hopper. Sometimes you may come upon a hopper that has paint in it. When a player is shot and drops all of their supplies their hoppers maintain their paint. 
Paint Grenades
This grenade is modeled after the real life squad buster. When tossed across the map it explodes in a few seconds and paint flies everywhere out of it. This paint is just like normal paint in the game and can eliminate opponents. This grenade is highly effective when people are camping in bunkers and other enclosed areas, where you can toss the grenade. They won't be able to get away from it quick enough. You can only carry one grenade at a time. To toss it use the bind, "tossgren". To drop a grenade and pick up another, use the bind "dropgren".
Smoke Grenades
The Smoke Grenade is similar to a flash grenade from Team Fortress. Once the grenade is activated it has a radius that expands for the life of the grenade. Once a player is within that radius their screen grows increasingly bright until they cannot see. It effectively blinds them but they can still be seen from far away. "Real" Smoke could not be effectively created due to the limitations of the engines, however this implementation is highly effective. To use the grenade bind a key to "tossgren".
Updated 8:14 AM 3/6/2004 to fix image links.