PCs can print color fine to HP Color LaserJet 5550, but iMacs can't

Color turns out dark and muted. Also, when a PC was setup to use a PS driver, color turned out poorly, too.

It seems that the PCL driver that the PCs are using works fine, but that the PS driver does not. We tried the generic PCL driver that comes bundled with Mac OS X 10.5, but it didn't print color.

Any suggestions as to how the Macs can somehow gain access to the PCL driver that the PCs are using?

Would installing Print Services for Mac on a Windows Server 2003 help in some way? Any other way to have the PC's assist the iMacs in some way?

Are you shure you have the right Mac drivers. Here are the HP specs:HP PCL 6, HP PCL 5c (HP PCL 5c driver available from the Web only), HP postscript level 3 emulation, native PDF printing (v1.3)
Connectivity
Connectivity, standard
IEEE 1284-C compliant bidirectional parallel port, USB 1.1 port, HP Jetdirect 620n Fast Ethernet Print Server in EIO slot, 2 open EIO slots, Jetlink port (for third-party paper handling devices)
Connectivity, optional
HP Jetdirect EIO internal print servers, HP Jetdirect external print servers, HP wireless print servers, HP Jetdirect EIO connectivity card for USB/serial/LocalTalk
Minimum system requirements
PC: Windows 2000: 300 MHz Processor with 64 MB of RAM; Windows XP: 233 MHz Processor with 64 MB of RAM; 180 MB free hard disk space for Windows, 1 GHz processor, 512 MB RAM (Windows Vista(TM) Ready, check user guide for minimum hard drive space), 160 MB for Mac; Mac OS 9.0, 9.04, 9.1, 9.2, 10.1; CD-ROM drive or Internet connection; dedicated IEEE-1284 parallel port
Macintosh: Mac OS X v 10.2, v 10.3, v 10.4, v 10.5; 4 MB RAM and 160 MB free hard disk space; AppleTalk, Bonjour or USB connectivity solution.

Apple did release a short time ago HP drivers update v. 1.1.1 for Leopard. Have you tried this as the 5500 sure is listed:-

Apple - Downloads - Application Updates - HP Printer Driver 1.1.1