Dib's black trenchcoat/boots and pale skin contrast work, just as Zim's olive skin and deep ruby eyes/uniform work. The black trenchcoat and black boots were made to go together and the coat looks weird without it. The vibrant blue of his shirt and blue shading on his glasses also worked perfectly for the character and brings perfect balance to the design. Even the graphic on his shirt was important, the grey of the graphic complimented his pale skin and the design gives you an idea of the kind of world he lives in and how Dib likes to present himself. He wants to look mysterious and cool, he emulates tacticool characters like Neo and JC Denton. His weird, clean haircut was important for that as well. He looks like Dib. He's a funny little character with a big ego, he gets himself in over his head, he takes himself too seriously for such a dork, and he engages in secret cyber networks.
The new Dib's color scheme adds a hodgepodge of vibrant and desaturated tones to the mix, and the new design is utterly afraid of using solid black. He has purple shading on his glasses, a more saturated skin tone, red boots, a purple coat, grey-blue pants, and a desaturated blue shirt. The messy haircut and sillier graphic on his tee, which shows he wears his fascinations on his sleeve, so to speak, is way too poppy and cheerful for a character like Dib in the world established by Invader Zim. It doesn't make a good parallel to the vibrantly colored Zim, it doesn't tell you anything about the world he lives in, it's not as distinct, and it speaks less about his personality. He doesn't look like Dib. He looks like a cute, colorful, happy, generic cartoon boy that likes cute ghosts.