Appearances are not edited on a per-model basis, if you're in an assembly, but on a per-appearance basis. Basically, you can have several different appearances, and apply them to different models. For examples, model 1, 2, and 4 might have appearance A, and model 3 and 5 have appearance B. Editing appearance B will affect both model 3 and 5. To edit the appearance of a single model, you need to apply a new appearance to it that is only used by that model. Then any changes you make to it affects only that model.
Also note that adding an appearance on a model in assembly mode will set that instruction only in that assembly. If you open the part in its own widow, or open a different assembly containing the part, the appearance will be unchanged. To edit the "inherent" appearance of the part, you need to do it in part mode, either by opening the part into a separate window, or by activating the part.
There are more subtleties to how this works. It's quite logical once you're used to it, but if it works differently from your previous CAD system, it's bound to be confusing.
By the way, you can change what is displayed in your popup and right-click menus in Creo. Right-click the popup menu and pick "customize", or choose "customize" from the right-click menu you want to change. Then you can add any command you want to it. Furthermore, you can record your own macros (called "mapkeys") and add them as buttons to your menus. So you could for example record a macro that creates a new appearance and adds it to the selected model, and you could place that macro on your right-click menu. Then you would have the behavior you're seeking. This is of course a bit advanced, for someone who just started working with Creo. 😄
