So. Let’s face it. Ever since the release of Halo 4, a lot of people have wanted to see the return of black undersuits. Why? Because the Mush of primary color between the Chest armor, shoulder armor, and upper area of the undersuit doesn’t look good. It works on certain armors because they have shoulder/chest pieces that are mostly secondary, which helps to remedy the issue. But the issue still remains on a lot of armors. The armors with "Space-diapers suffer the most. Look at Mark VI gen-1. The Diaper covers up what little bit of black undersuit we still have, making even more of a primary color mush. It looks pretty bad. So why did 343 create the colored undersuit?
The Colored undersuit was part of a tradeoff. Remove the space diaper, and fill in that void with the primary color. And in a sense. That tradeoff works. A lot of armors 343 designs can afford to have less lower torso armor, and no space diaper, because the undersuit fills in the void that would normally be there. The issue, however, comes in with the shoulder area, as I already explained.
Take a look at this album for reference of what I’m talking about. As you can see, some of 343’s armors would potentially suffer from the black undersuit, and might look awkward with no lower body armor. But other armors, like Helioskrill, with even some lower torso pieces could really benefit from the black undersuit, especially armors like Mark IV, V, and VI gen 1. And even some of the armors without space diapers would look fine anyway, such as Noble. Palmer int he Campaign has a black undersuit. Oddly enough this is white int he cutscenes, but the in game model has it black. She does just fine with it. The way I see it, either way, some armors are gonna look awkward with one type of suit or another. So 343 should just give is the option. Not to mention that they have action figure, such as the Mark VI Gen 2 in this picture, that have black undersuits. 343. Why do you torture me like this?!
The last image in the album I linked is somewhat of a custom undersuit on the Warrior armor that uses black shoulders only while leaving the rest colored. I think it looks pretty cool.