What kind of figure of speech is each of the following quotes?"Stars, hide your fires" "I'll gild the faces of the grooms withal;for it must seem...

Both quotes also reveal imagery: vivid language that represents objects, actions, ideas.

In particular, the first quote reveals imagery connected to the ideas of "appearance versus reality," "lightness and darkness," and "heavenly bodies."  And if you read on, "hand" and "sight" imagery:

Stars, hide your fires,

Let not light see my black and deep desires;

The eye wink at the hand; yet let that be

Which the eye fears, when it is done, to see.

In the first soliloquy, or aside, Macbeth reveals the duality of images: the hiding of stars' "fires" that represent "black and deep desires."  It's external (appearance) vs. internal (reality) imagery.  As well, the "eye" (knowledge) vs. the "hand" (action).  Later, after killing Duncan, Macbeth will not want to go back into the chamber and look on the crime his hand has committed.

The second quote reveals the same "appearance versus reality" duality: hiding or transferring guilt.  In this case, it is the external face showing guilt when, in fact, guilt is internal.

In both quotes, the Macbeths will never be able to hide the stars' fires, never be able to wash the blood from their hands, and never be able to hide the guilt from their sleepless nights.