Rep Power: 0

I'm having some trouble with my 2000 Honda Civic Ex automatic 2-door coupe. My car is overheating. This problem has been occurring for about a month now. I've noticed the tank where the anti freeze goes is kind of dirty and I'm changing it every so many days because it seems like the anti freeze keeps disappearing or it'll be very little but a different color. I check under the car but there's no fluids on the ground. The car will usually overheat when it's idle, on but sitting in one place. It also begins to overheat if i drive it for a long period of time. I recently got a new thermostat put in. I also recently got an oil change and they checked for leaks and other issues. I do have a small leak where the distributor is. I don't know if maybe that could be causing the car to overheat or what? The oil is fine, no odd colors, no thick consistency. The transmission fluid is fine as well. I recently also flushed the radiator but the problem is still occurring. I just want to know why the coolant over flow tank continues to look dirty and what could be causing my car to overheat like this?

 
Rep Power: 211

Hate to say it and I just went through (and maybe still) one but, it might be a blown head gasket causing the overheating. However, try to rule out other possibilities first.

Is radiator fan switching on?
Any small leak at water pump location?
Any leaks near the heater core inside your car? You will most likely have to pull the carpet back to see.

You can rent a block tester and buy test fluid ($9) from most auto parts store which tests for hydrocarbons in coolant..if present blue dye will turn yellow or green which means head gasket leak:

or like this: