Yeah, there was something else I meant to mention but forgot about.
They refer to the pilot's personality because their personality seems to dictate how their MP gauge looks. Kai, Beecha and Otto are a tad cowardly, so it's easy to drop them into low tension, and hard to get them up to high tension. Jerid, Judau, and Domon seem pretty confident (arrogant, maybe), so it's harder to reduce them to low tension than to raise them to high tension. Audrey, Shakti, and Relena are pacifists, so they start off in low tension and need to put in work just to get to normal. Cool-headed characters like Quattro and Zechs have a large green area, over-emotional characters like Kamille and Shinn have a small green area, and unstable characters like Four and Clotho have barely any green area. Berserk characters like Ple Twelve, Wing Zero Quatre, and Innovade Louise are just pure red.