The increased shield capacity, that the Overshield provides, should have to be taken out by damage from the enemy team, not by decaying over time. If the double layer OS didnt decay, for one minute after pick up, it would encourage the enemy team to engage rather than avoid while the OS decays on its own. Sixty seconds after pick up, the 2x OS, or whatever is left of it, rapidly drains away similar to how it fills on pick up. The increased OS charge up speed minimizes the invincibility period after pick up, down to a brief moment, while not removing it entirely.
Removing OS decay serves to remove unpredictability when engaging an OS bearer. It is difficult to determine how much OS a player has left with decay so when you engage him you cant be sure what the best approach is. If a team has good communication, they can know exactly how much OS he has left by calling out how many shots they put into him. Increasing the player's ability to effectively communicate with his teammates allows the team based skill gap to grow. A team that knows exactly how much OS remains shielding their target can develop, and deploy, more effective attacks against him.
2v2 gametypes should use a single layer OS, for 200% shield capacity, while 4v4 and up should use a double layer OS, for 300% shield capacity. The reason for this is because as player count goes up, so does the effectiveness of teamshot against the OS. An enemy OS user should force the opposing team to work together to take him out. The exception to this rule, of requiring teamwork to take out an OS, would be through the use of an Anti-Overshield Weapon like the overcharged Plasma Pistol. Unlike the double layer OS, the single layer OS rapidly drains after 30 seconds instead of after a full minute.
The color of an Overshield pick up corresponds to the number of shield layers it gives. A green OS pick up gives one extra shield layer, shown as a green bar over the players default blue shield bar. A gold OS pick up gives two extra shield layers, one gold bar and one green bar over the players default blue shield bar. The color of an Overshield users glowing aura corresponds to how many shield layers he has. A gold aura means some of the second extra layer is left. A green aura means some of the first extra layer is left. The colored aura disappears once all extra shield layers have been depleted.
If the Overshield's respawn timer is set to match its duration, 30 seconds for a single layer and 60 seconds for a double layer, it encourages the team that didn't grab it to engage the enemy who did because if he still has an OS when the next one comes up, he will likely secure that too. The strength of an Overshield does not continue upscaling past 4v4, like a 3x OS for BTB, because a power up that strong would need a respawn timer too long to effectively promote map movement.