How to assign different faces of the same object different materials? I have subdivided a plane into 50 x 50 faces, 
then I random selected some of those faces and saved them as a group, 
I hid them with h and random selected among the other faces and made them another group. 

I repeated this a couple of time, so that I had one objected which contained several groups of faces. 

From this point on, I tried to select one group and assign to them one material and to another group another material and so on...basically to achieve a floor of tiles with different kind of tiles.

But, it did not work.
Click "+" to add a new material slot
Set the material
Select the desired face(s) and make sure the added slot is selected
Click "Assign" button


You should see it in the viewport (solid) now