The default setting for Generic Postscript printing under Snow Leopard is black and white only. We use a Dell Color Laser 3110cn [sic], so it should be possible for a Mac to print under colour.
There are no settings in the Cups configuration (http://localhost:631) to switch on colour printing. PCL 5/6 supports colour printing and is supported on this model, but it is wading through treacle slow. Fine if you want your CUPS jobs to take 10 minutes to print, otherwise avoid.
To get Colour postscript printing to play nicely, setup the printer as normal using either http://localhost:631 or System Settings->Printers->Add new printer.
Select Generic Postscript Printer.
Open a Terminal using Applications->Utilities->Terminal.
$ cd /etc/cups/ppd
$ sudo vi *.ppd
Dell Color Laser 3110cn
Remove the following two lines:
Restart CUPS (or restart your Mac if you fear the command line):
ps -ef | grep cups[d]
Identify the cupsd process and the PID - in my case this is 491
kill -HUP 491
Try printing a colour page. It should process quickly and be in colour.