Actually I do it programmatically for every installed system, deleting the XCenter and creating a new one with the following setup string:
POSITION=TOP;BORDERSPACING=0;REDUCEDESKTOP=YES;WIDGETS=XButton(),TasksWidget(),ObjButton(OBJECTHANDLE%3D<XWP_LOCKUPSTR>%3B),ObjButton(OBJECTHANDLE%3D<XWP_FINDSTR>%3B),ObjButton(OBJECTHANDLE%3D<XWP_SHUTDOWNSTR>%3B),Pulse(WIDTH%3D154%3B),Tray(BGNDCOL%3DCCCCCC%3BWIDTH%3D134%3BCURRENTTRAY%3D0%3B){Tray 1[ObjButton(OBJECTHANDLE%3D<WP_DRIVES>%3B),ObjButton(OBJECTHANDLE%3D<WP_CONFIG>%3B),ObjButton(OBJECTHANDLE%3D<WP_PROMPTS>%3B)]},WindowList(BGNDCOL%3DCCCCCC%3BTEXTCOL%3D000000%3BFILTERS%3DSysBar/2 Pipe Monitor#~^°@lSwitcher#~^°@FTP server%3B),Power(BGNDCOL%3DCCCCCC%3BTEXTCOL%3D000000%3BWARN%3D15%3BREPEAT%3D5%3B),ObjButton(OBJECTHANDLE%3D<WP_ASSISTANCE>%3B),ObjButton(OBJECTHANDLE%3D<C:\Sys\bin\calendar.exe>%3B),XWLANMonitorWidget(),Time(BGNDCOL%3D000000%3BTEXTCOL%3D00FF00%3B),AN_RemMedia();OPEN=26599;OBJECTID=<XWP_XCENTER>;
but you can also do it by hand, dragging colors from the Solid Color Palette (folder Appearance under System Setup) to drop them onto the Time widget. Ctl+drag to change the foreground, simple drag for the background.