I want to change light fixtures, but the wires are red, black (already capped off) and a white wire... do I uncap the black and use the red as the grounding wire?

I have a red, black (already capped) wire and white wire in the box. The unit has a green, white, and black. What hooks up to what... if I understand correctly, I hook the white to the white, the black to the red, but what happens to the green?

Please clarify. Are you saying that you have an existing fixture connected to the red and white wires, that there is also a black capped off wire, and now you want to replace that fixture with a new one? In general red wires are for switched power - is the lamp controlled by a wall switch? Red is NEVER used for ground or neutral, so connecting a fixture to both red and black would not work. Black is generally for constant hot and white for neutral. So if the old fixture is switched and connected to red and white, the new fixture should be as well. But again it would help if you could better explain the existing situation.

EDIT - you made your second post as I was composing the above response to the first. OK, this helps. First of all yes, you attach the black lead of the fixture to the red lead (which I assume comes from a wall switch), and white to white. The green wire on the fixture is a ground, and should be connected to the electrical box itself (assuming the box is metal, not plastic). It ensures that the metal parts of the fixture are always connected to ground to avoid the possibility of shocks to someone who touches the fixture. To attach the green lead to the box you can use either a grounding screw (most boxes have a threaded holes to allow this) or a grounding clip that attaches the green lead to an edge of the box opening.

Okay, the ceiling has a bundle of black wires already capped off, a bundle of white and a red wire. Yes, I want to switch light fixtures. Now, the old fixture was hooked as follows, the white wire was hooked to the bundle of white wires and the black wire in the fixture was hooked to the red wire. My new fixture (not a dimmer light, just a normal track light) has a green wire, red wire and black wire. If I hook the white to the white, the black to the red, what do I do with the green?

Okay, I've read your post and so I need to purchase a green grounding wire to hook the lights up and I continue as I mentioned? Black with red and white with white?

Lima,

Are you sure that there are no bare copper conductors in the ceiling? If there is at least one bare conductor, the green wire from the fixture would connect to the bare, equipment grounding conductor.

If you look at your ceiling box, there should be a Black - White - Bare cable that feeds the box.

The White from this cable should be grouped with the white from the fixture and any continuing circuit if one is present. The Bare wire, should also be grouped with the fixture ground (bare or green)and any continuing circuit.

Now, if there is a three conductor cable, from the main feed to the wall switch, the black from the main feed will connect to the black going to the switch. The red, will be the return from the switch. It connects to the black on the fixture. The white will be grouped with the other white conductors.

At the switch (if this is just a simple snap switch) the white conductor will be capped off as it is not needed. Current code requires that "Neutral" conductor be present at each switch outlet in case it is needed by a specialty switch such as a pilot lit switch.

You said;

My new fixture (not a dimmer light, just a normal track light) has a green wire, red wire and black wire.
Then you said;

If I hook the white to the white, the black to the red, what do I do with the green?

Earlier you also said;

The unit has a green, white, and black.

Did you mean to say that the light fixture has a green wire, a white wire and a black wire?

If so, you connect the white wire from the light to the bundle of white wires. You connect the black wire from the light to the red wire in the ceiling. You connect the green wire to the bundle of bare wires in the ceiling. If there are no bare wires in the ceiling you put a wire nut on the green wire and leave it unconnected.

To say it another way. You connect the new fixture just like the old fixture. If there is no place to connect the green wire, no bare wires, you leave it unconnected.

The green wire should be long enough to connect to the bare wires in the ceiling if you have them. You should not have to purchase any green wire to connect.