FX-Foundry has a pretty decent plug-in to simulate infrared.
After install you will find it under FX-Foundry/Photo/Effects/EG Infrared Simulation.
Then you just need to create a mask on the B&W layer copy, and fill it with black.
Now using white, paint in your color.Now you can change that color to anything you like by choosing the mask and selecting all the white in the mask.You will have to do this for each color (a separate mask).
Example (EG_Infrared with mask)
Personally i use Tintii, the freeware, and get very close to the same results.
Tintii can be called from Gimp through Rob A's Shellout.py or the Photo Shop 8bf plugin through the Adobe plug-in initializer PSPI.exe.
Now i just open the saved Tintii image in Gimp and go to Colors/Curves and play with it a bit
Pretty easy with Tintii.