You can choose which renderer to use for UT. The default is D3D. An alternative is OpenGL. You will find the opinion as to which is better to be split down the middle.
IMO, OpenGL is better. It is smoother and brighter. If you want to try it, follow these steps.
A. The basic install of UT comes with an OpenGLDrv.dll, but it is not the lastest, so we won't use it. Go to your UnrealTournament/System folder, find the OpenGLDrv.dll file, and rename it something like OpenGLDrv.dll.ORIGINAL
B. Download this version of the dll:
http://www.birdieman.com/downloads/utOpenGL36.rar, unzip it and put it into the UnrealTournament/System directory.
(you need to write down this next step because you have to go open UT now. After this step you will be able to read this again)
C. Start UT. From the menu choose Options/Preferences. The very first option is Video Driver, and there is a button that says "Change". Click this button. This will give you a message "This option will restart Unreal now, and enable you to change your video driver. Do you want to do this?". Click Yes.
D. This will open a UT window. Find the option that says "Show All Devices", and click it. This should add more items to the list to choose.
E. From the list, choose "OpenGL Support", and then click Next, then Next again, and finally Run.
You are done. If you don't like it, you can basically follow the above steps to change it back.