In case you haven't already done it, The HP Universal Driver needs to have certain settings in the "Device Settings" tab of the Printer Properties in order to ensure that jobs come through in color (or not).
The ONLY way I've been able to get it to work consistently, is to have one printer setup for BW and another printer setup for COLOR. In the Device Settings tab of the Printer Properties, set "Device Type" to 'Monochrome' if the printer setup is for BW. set "Device Type" to 'Color' if the printer setup is for COLOR. If using the PCL drivers, there may be a "Color Printing Mode" item. Set "Color Printing Mode" to OFF for BW setups and set to ON for COLOR setups.
Do not use "Auto Detect" or "Automatic" on either setting or else you'll have users trying to print in color and the job comes out in bw instead. Sound like the problem you're seeing?
By the Way, the settings in "Device Settings" applies to all users of the computer. Make sure you set the settings at the server, then check to see if the end users are seeing the setting change. If not (they could be using "Local Printer" setups) you may need to also set the settings at the user's computers (depending on how your organization has users connect).
I ran into something like this a awhile ago and the easiest solution was one queue for Color one for B&W