I've created a room in blender (one floor plane+ 3 wall planes) by duplicating the floor plane. However, when I change the floor plane colour in materials all the other planes change also. How do I stop this and just assign a colour to one plane in view in 2.8.
There are two possible ways to duplicate, one is duplicate object in Object Mode and another is duplicate faces on Edit Mode. When you duplicate objects, material also is duplicated applied automatically same as object from copied one.
To avoid this you have to add different material to each object from material Panel. Click on X in material panel and delete old material, and then click on New to add new material to object. Do this to each object you want to have different material.
Let's check another possibility: If you have duplicated faces in edit mode. In such case add more material slot in same material panel by clicking + button beside material name; Then click New. After that select face, select material slot you want to be applied and then click on assign. Now all faces are having different material.