From the parts list for a 3.3 revo (also in the manual)
5435 Spring, shock (GTR) (2.6 rate Yellow) (1 pair) ............................... $6.00
5436 Spring, shock (GTR) (2.9 rate white) (std. front 90mm) (1 pair)......... $6.00
5437 Spring, shock (GTR) (3.2 rate orange) (1 pair)............................... $6.00
5438 Spring, shock (GTR) (3.5 rate green) (std. rear 90mm) (1 pair)......... $6.00
5439 Spring, shock (GTR) (3.8 rate gold) (1 pair) ................................... $6.00
5440 Spring, shock (GTR) (4.1 rate tan) (1 pair) ..................................... $6.00
5441 Spring, shock (GTR) (4.4 rate black) (1 pair).................................. $6.00
5442 Spring, shock (GTR) (4.9 rate silver) (std. front 120mm) (1 pair)....... $6.00
5443 Spring, shock (GTR) (5.4 rate pink) (1 pair) ................................... $6.00
5444 Spring, shock (GTR) (5.9 rate blue) (std. rear 120mm) (1 pair) ......... $6.00
5445 Spring, shock (GTR) (6.4 rate purple) (1 pair)................................ $6.00
The bigger the number for the rate, the stiffer the spring is.