No, you can't use set analysis in the RGB function. If you have a table with colors per week in red, green and blue values you can put those into the RGB function
load * inline [
WeekNo, R, G, B
1, 255, 0, 0
2, 255, 128,0
3, 255, 255, 0
If you have a table as above you can use WeekNo as dimension and an expression like RGB( R, G, B) for the color expression for dimensions and expression colors.
If you need more help please post an example file that explains what you want.