I've always thought that Red (10-148-2) was Williams most powerful plunger spring, the parts listing for High Speed shows it specifically on the page for playfield parts, even though it's in the cabinet.
I think the standard rating was Blue, the next down was Green, for games with a short entry shot, such as Cyclone
The two weakest ones I'm aware of are Brown (Twilight Zone?) and White (Roadshow- right side?), two of Pat's later games
For FunHouse, the manual states that both springs are 10-148-1, but ipdb has a text file 'Parts list' which details two different ball shooter assemblies, one using 10-148-1 Silver and the other -4 Green. Looking again at the manual, the drawing of the cabinet on page 1-4 shows the left shooter as B-7592-2, the one listed in the parts list as having the '148-1' (Silver, .035) spring.