A gray warning symbol means that you are using a font that is not installed in your system, whereas a yellow warning symbol means that the document you have opened was created in a different 

Which image(s) belongs to the red channel if the source image is in RGB mode?

You can apply Brush, Eraser and Clone Stamp on vector based layers

What are the names of the tools marked in red in the image above?

The Mode pop-up menu of Puppet warp tells Photoshop how stretchable you want the mesh to be.

..... is a process in which Photoshop responds to your size-change

Shortcut for the Eyedropper tool?

Shortcut key for the horziontal type tool, Vertical type tool, horizontal type mask tool and the vertical type tool?

Shortcut for the eyedroper tools is

How do you turn a standard layer into a background layer?

Pressing Cmd (Ctrl) + Z Toggles the last history state in Photoshop. What do you press to keep undoing?