New material to selected face

Hi,

I’m having a hard time right now assigning a different color to one selected face. I have three materials: red, green, and yellow (I want to create a Cornell box), but after assigning a yellow material to the box, when I select one side in order to give it another color, the color of the whole box changes. I found a thread to this problem, but for me the solution doesn’t work for some reason. What can I do?

TIA,

Claus

Edit:
“but for me the solution doesn’t work for some reason. What can I do?”
Could stop asking vague questions and provide support files because “it doesn’t work” is useless information. Tutorial linked in my signature shows why and how to do that.

Could also use official blender from the website instead of Ubuntu repository version when asking questions and troubleshooting. Repository versions are different and have problems specific to them, and problems with those belong to distribution support.