Custom and Web colors just translate to numeric values in RGB space, the Web colors are named for convenience by MicroSoft: they should
look the same everywhere: but, they may not: my monitor may make your purple look rosy :)
System Colors means that color used will be based on the user's selected Themes and color settings for typical items like TitleBars, and ScrollBar color, etc. So they will vary from machine to machine.
Use System.Colors when you want something (usually a UI element) to look consistent with other applications' UI's appear on the user's machine.
Highly recommend you read this Wiki page [^
] for more understanding of what Color names mean in the browser, and in CSS3's X11 specification.
Microsoft is using the CSS Level 3 specification for its named colors.
For information on MS named colors:[^
And a good overview of MS' evolving its use of color specifications here:[^