The Mixels logo is a giant, super-ultra MAX that requires every last Mixel in existence. Leave one out, you'll just get a regular-looking max (I guess like the Ultra-Miximum Max) the size of a galaxy.
Likewise, the Nixels logo requires every single Nixel in existence to combine. Since it's not done with cubits, leaving one out might just make it look chipped. (The eye is made of un-shelled Nixels!)
Also, the reason the Mixels logo is so yellow is because there are so many yellow Mixels.