Here is the email I rx-ed from George regarding the spring colour vs tension and the shims.
Yellow: medium/soft stiffness
Blue: medium stiffness
The flat discs or shims are just for that. Pre-loading. We've found
that some of the WRX's respond differently to the new valves. Not sure why, it just seems to be an idiosyncrasy of the brand. Anyhow the shims allow the user to "fine-tune" the spring tension in the event that the spring alone is not working properly. This happens relatively infrequently but enough for us to offer a solution for when it does.
I don't have any printed material on how to tune and service the
valves but it's a very easy operation. The top of the valve simply
twists off. The springs and piston on found underneath. The shims are placed in the top of the piston and then the spring is put back on top of the shims. The top is then screwed back on, the valve re-installed and the user can then test the results. Usually this will take a couple tries but once it's set, it will not need any further adjustment unless the owner makes changes to the car that effect the valve such has increased boost.
I hope this has answered your questions. Please feel free to
contact me if you need any more information..
Director Sales & Marketing
Forge Motorsport Inc.
Phone - 321-689-0972
Fax - 321-697-0060