The default system font is used. On Win32s, you will have to go to another window instead. Don't forget that you must tell Windows 3.1x to keep executing a DOS command in the background while you switch back to Vim. On Win32, the normal DOS colors can be used. Additionally the system configured colors can also be used.
Case is ignored.
This file is well known from X11. The four fields are separated by spaces. You can get an rgb.txt file from any X11 distribution. The Win32 GUI can use familiar Windows components for some operations, as well as the traditional interface shared with the console version. There is no option to change this.