I suggest building a table for the colors instead of handling it in expressions. You're surely going to want these colors in multiple places, which means repeating the expression. You could create a variable to hold the expression and use the variable, but I'd build a color table instead. I just think it's cleaner, and would allow you to load colors from an external source like a spreadsheet if desired. It requires script access and a reload to change, of course, which might occasionally be an issue.
The attached example allows you to explicitly assign colors to customers by RGB values, and then automatically builds random colors for any customers you didn't assign. You don't want the random color part in this case, so skip that and make certain that all colors are assigned.
The color expressions in these charts are also more complicated than you'd likely need. All you'd need is rgb(R,G,B) in most cases. These charts were doing more complicated things like highlighting the weekday data.
Edit: Ooops, sorry, I see you're in personal edition. I'll make another edit to tell you all the relevant information.
Edit 2: Actually, rather than describe the example I posted, let me just give you what I think you should do. Consider a spreadsheet instead of an inline load, but here's the idea. In the script:
[Colours]:
LOAD * INLINE [
colour,R,G,B
Red,255,0,0
Pink,255,150,150
Green,0,255,0
etc.
];
Then in the chart, in the background colour expression:
rgb(R,G,B)
