After installing Citrix Presentation Server 4.x Hotfix Rollup Pack 1 or 2 you are able to enable 32 bit icon color depth in Farm Properties
If you don’t see this option in the Farm Properties you should dowload and install the latestest AMC Presentation Server Extension 4.6.1
Update :
Remember that you also have to upgrade to Web Interface 4.6 and AMC on your IIS server.