Left4Dead/Left4Dead2

I am trying to run l4d and l4d2 in Parallels and they run fine but the graphics are really dark and it makes the game nigh on unplayable. My specs for the system are: 2.8 ghz core 2 duo, 4 gb of RAM and the graphics card is NVIDIA GeForce 9600M GT. Any help would be appreciated.

Playing games

I would use a boot camp to play games and then it can use the full power full of the system without vitualisation getting in the way,

Any other options?

Not really. The latest version of Parallels has better 3D support than earlier versions, and you didn't tell us which version you are running. An upgrade would make sense if you haven't already. But the simple fact of the matter is that under Parallels and Fusion, you are running a software-emulated graphics adapter. Under Boot Camp, you are using the real thing. That's going to be better. Your call.

Definitely install Boot Camp 3.1 ...
And other just stupid advise .. check brightness in your game settings: D

L4D4 shouldn't very bright .. this game is dark and gloomy, nothing to do with the sun and bright colors)

Valve have announced they'll be releasing a Mac port in the not too distant future, so you'll be able to ditch the VM then...