CrossOver issues this warning when the screen is running in thousands of colors mode or less (that is 16bpp or less) instead of a mode where it can display millions of colors (24bpp). Running with a low screen depth can cause display issues in Windows applications.
You should change your screen / desktop configuration to run in millions of colors (24bpp). The exact way to do that depends on the platform and on the desktop environment so only take the following instructions as a rough guide:
Fedora Core : Start Menu -> System Settings -> Display Mandriva : System -> Administration -> Control Center -> Hardware -> Configuring the X server SUSE : Start Menu -> Control Center -> Hardware -> Graphics Card and Screen Ubuntu : System -> Preferences -> Screen resolution
If the above does not match what you find on your computer, please refer to your system's documentation for how to change the screen color depth.