I have created a simple small (10 metres long) girder bridge which is made up of a base, hand rails and the vertical girders between the base and hand rails all using separate using mesh cubes. I firstly set the color of all of the object to a galvanised color and then decided to color the walkway (top face of the base) near black to represent a non slip rubber surface. I have added another material set to near black color and then attempted to add it to the top face of the base. With 'face select' selected it doesn't matter which face I select in edit mode as soon as I then select my 'black' material ALL of that part of the object changes color. How can I apply it only to one face.
You changed the material in the first slot to a different material - but you have to keep the first material if the rest of the object should stay as it is.
The way it should work is: you have to add a second material slot by pressing the Plus button right to the material slots. Then create or select a second material for the second slot. Now if you have two materials, usually the first material is applied to everything until you tell Blender differently. So, to change a single face for example, select that face in Edit Mode, go to the materials tab, select the material this face should have and click the Assign button below the material slots. This assigns the selected material to selected faces.