It's posible to emulate CGA graphic card in a computer with an Hercules Graphic Card on the 90% of the cases.
You only have to run one of the emulation programs before run your CGA game or application.
 

A large Hercules Graphic Card

A CGA Card

Download CGA emulators for Hercules computers DOWNLOAD CGA Emulators
There are 4 choices to run your CGA game, and you have to try and test it.

 The best choice is called EMU0.EXE .
 Type:
 EMU0 80
 At the DOS prompt to change current Hercules mode to CGA emulation.
 To run emulation with auto-boot disks, type:
 EMU0 80 B
Then the computer will load the emulator and simulate a boot up.
If it doesn't work try the others emulators.
Another choice is EMU.EXE
The next one to try is CGA.COM
And if none of this works, you could try to run:
E1
E2
E3
Run them all secuentially, and then run your CGA software.

I used this emulators to play games on my Amstrad PC 1640 MM (aka 6400 in USA), and usually EMU0 80 ran nice, but on some cases 10% aprox I had to try the others options