I thoroughly agree that this concept art was also a test for some things in Flying Battery. As someone said earlier, it makes sense that they used a basic concept and tried variations of it to get an idea of the style of the level. They obviously kept some blue stuff from the colour test by the time Flying Battery came along in Sonic and Knuckles.
Some of you are saying that it can't be because the colours are different. Well, Sonic 3 was meant to have Flying Battery in originally, and I have this to say;
That's the same colour as this;
That must mean that it's at least an early re-used concept, because hasn't a Flying Battery save screen been found in Sonic 3 hidden away, which has the final colour scheme?