For Windows Vista users, you already have the latest remote desktop clients, so you can make sure that your connection’s “Options” is using “High Quality (32 bit)” color in “Display” tab, and using “LAN (10Mbps or higher)” in “Experience” tab.

If you are using Windows Vista Professional or Windows Vista Premium, you may following similar instructions of “Enhance Terminal Services’ Color Depth by MMC” to make your connection become true color (24 bit) connection. If you are using Windows Vista Home Edition, you can use registry editor to add a key as following:

[HKEY_CURRENT_USER\Software\Policies\Microsoft\Windows\Terminal Services]
“ColorDepth”=dword:00000004

Or you can download the registry file VistaTerminal24bitColor.reg and merge it into your system.

If you are using Windows XP or 2003, you need to upgrade your Terminal Services Client to 6.0. And then following the above MMC enhancement instructions or add a new key in registry:

[HKEY_CURRENT_USER\Software\Policies\Microsoft\WindowsNT\Terminal Services]
“ColorDepth”=dword:00000004

Or you can download the registry file XPTerminal24bitColor.reg and merge it into your system.

References:
http://www.j79zlr.com/gphome.php