Actually, it's a LOT more complicated than that, so lemme explain.
Originally, SRB2 had four shields: Basic (blue), Elemental (green), Attraction (yellow), and Armageddon (black). The Elemental Shield was supposed to protect you from elemental damage and from drowning, so it would protect you from ALL types of damage. You'd jump into the lava and you'd be immune. The green was supposed to signify nature, if I recall.
Now, personally, I was never a huge fan of the elemental shield originally (I've since warmed up to the idea, but it's too damn late now). I preferred a more Sonic 3-style system where there is elemental damage that you could be protected against by the elemental shield. Also, having a green shield protect you from fire looked kinda weird.
Hence, the Inferno Shield (red) was added at my request. Later on, someone added electrical damage that could be avoided with the Attraction Shield, making the three main shields in the game act like Sonic 3.
However, that left us with a strange situation. The "water shield" was still green. Blue was still taken by the Basic Shield. To fix this, I coerced AJ into making the Whirlwind Shield (white), replacing the Basic Shield, and moving the renamed "Liquid Shield" to blue. I still consider finally getting AJ to make me a double jump shield to be one of my favorite things I've done in the game. I only harassed him to do it for like, 2 years =P
So, that's where the Elemental Shield went. I killed it. Honestly, looking back I'm not so sure it was a good idea, but we got the Whirlwind Shield out of it, so it can't have been all bad.