Quite possibly, if the wall and pillar are separate mesh objects, they are using the same material. When you “Change the color”, you are not changing the color of the object, you are changing the color of the material that the object is referencing. Anything else that is using the same material will change color as well.
- Select the wall object
- Press F5 for shading
- Look at the Links and Pipeline panel
At the top left of the panel, you will see the material name. Immediately to the right of the name, there may be a number. If there is, the number is the number of objects that are using the material. If you hold your mouse cursor over the number, you will see a tooltip “…Click to make single user copy”. Clicking the number will make a new material that is only referenced by the object that you have selected.
Best of Luck!
in the editing panel, go to links and pipeline, then click new on the material until you have 2 materials, then go in to each of the objects edit mode and assigning the vertices you want to that material, they dont have to be different objects if you use this method