Duplicating surface of cube adds a circle to it I have recently started learning 3D modelling in Blender and I am making model of a Rubik's Cube.

Here is what I have done:


Added Bevel and Subdivision Surface to the default cube for smoothing its edges.
Duplicated one face of cube so that I can color it.


But when I duplicate the face it gives me a circle on top of that side instead of exactly duplicating the face. What am I doing wrong?

Here is what I am getting: 



EDIT: I am duplicating the face because I want to keep the grey boundary while apply the material in central area. I solved the problem mentioned above by adding 4 loop cuts to the face. But when I apply color on faces they all appear kinda jittery when I rotate the cube. Can anyone tell me why is that so?
This is because when you duplicate the face, it is now a flat plane and the subdivision surface modifier is treating it as a separate mesh and rounding it.



To get around this, you can disable or hide the modifier while working by clicking the eye icon  on the Subsurf modifier.

If I also understand what you are trying to do, it's also worth noting that you don't need to duplicate the face to apply a material to it or to modify it specifically. Objects can have multiple materials in Blender. You can go to the material tab, select a specific face, add a new material and apply it specifically to that face by clicking Assign.