Surfing through the list archives for an answer to why can't i use more than 16 bit Color Depth when connecting to a WinXP TS, i found a lot of threads opened on this matter and a lot of people having this problem. I finally sort it out on my own, but i thought i'd post a relevant message to this list for all those people encountering the same issue, or for future searches.
Here are some notes to consider:
1. You can NOT get a higher color depth/resolution from a session than your video card on the client computer supports, even if the video card from the server computer supports it. (Obvious, but just to make sure it's understood)
2. The OS running Terminal Services DOES count.
3. Windows NT/2000's Terminal Services does NOT support a color depth higher than 16 bit. If you run these OSs and want better, don't bother, you won't get it.
4. Windows XP's DEFAULT Maximum Color Depth is ALSO 16 bit. This explains why when you try to use a 24 bit color depth option with rdesktop you will get WARNING: Color depth changed from 24 to 16 or something to that extent.
Here's the solution for Win XP (also 2003 server i guess):
Go START > Run and type in: gpedit.msc
The Group Policy Editor will open. In the GPE go
Local Computer Policy > Computer Configuration > Administrative Templates > Windows Components > Terminal Services
Once you click on Terminal Services look in the right TAB and search for Limit Maximum Color Depth . It will probably have status Disabled by default. Right click on it and go Properties. The Properties window will open up. On the Setting TAB select Enable and at Color Depth select whatever you want. If you select Client Compatible you will allow terminal services to read the color depth setting from the client, so you can control it from the client. If not you can set whatever color depth you wish from that list. Yes 24 bit is maximum. As far as i know the RDP , even 5 has not enabled support for 32 bit color depth yet.
Good luck and hope it helps.
Lucian Constantin (Skello)