I used P and from the pull down menu seperated a selection as indicated in the picture. I labeled the new object EAR and added a material I made the object EAR black and added backface culling by individually setting the direction of normals in my transform window while editing the ear object. I am also using a mirror modifier. As this is supposed to be a harbor seal. It only has ear holes no ears on the outside of its head. The back face culling is merely for editiing purposes so that I can set the placement of the ear correctly while I work on making vertices groups and shape keys ect. I can select both objects using shift click in the Outliner. However I am having trouble getting the vertices select mode to show up on both objects at the same time. only one of them at a time. I know that I can join the two objects into one by selecting them in the outliner in this way by pressing cntrl J. What I don't know is whether or not, this will allow me to keep my original diffuse color. If so how do I organize both colors? because both objects will then become one in my Outliner right? so can I join the vertices while keeping seperate objects? I am almost sure this is going to pose a problem simply hitting cntrl J. So I am going to wait for a while to see if anyone is able to answer my question. I have some smoothing to do on my mesh in the meantime, and I can work on other things I suppose. Also it may be relevant that I have added vertices and moved them around just a bit after seperating the selection. I hope I am not too confusing, here is my harbor seal so far.
Blender only allows you to edit one object at a time, so yes you can only see and select the vertices of one object.
Every face in a model can use a different material, if you join multiple objects, the combined object will have multiple materials available and each face will continue to use the same material it used when it was in a separate object.
In edit mode you can assign a specific material to the selected faces.
If you join the multiple objects into one, you can use the by material option in the separate menu to split each into it's own object based on the material it uses.
So yes you can join multiple objects and they will keep the materials. You can easily re-separate them. You can also create one object, assign different materials to parts as you model and then separate them based on the material when you are finished.