Thanks for the input Nomad. 

@abc : You could try adjusting the saturation with the steps given by Nomad.

since you say that your'e seeing the images having good colors on your camera, and dark in your PC monitor,  Im pretty much sure youre camera LCD is set too bright. try reducing it too.

@ Nomad. appreciate your help on the steps to adjust the LCD brightness on the D90..

Goodluck.