This would be a big win for mages, and is primarily the best thing that could help casters. I would do more than just coloring the Z plane if possible as depth is not always the problem.
Personally I think a very useful feedback for mages would be similar to that of bridge simulators. Submitting a spell would show levels of “tension” where tension represents how incorrect or correct each part of the drawing is. The levels of tension are visually represented by hues. Blue being perfect, red being horrible & the hues shift everywhere in between blue/green/yellow/orange/red. Example:
If the player sees red, they can start manipulating that part of the spell until it becomes orange, then yellow, then green, then eventually poof! It casts. Muscle memory and practice are still required to actually use any spell consistently in combat.
To avoid showing feedback for a similar looking spell, on each page in the mage book a player can select a single spell. This would cause the feedback system to ignore all other spells when it evaluates the “tension” of the spell submission.
Opening the spellbook & selecting a spell could be a way to enable this feedback system & closing the book would disable it.
To clarify, the player opens the book, selects a spell, makes a drawing, submits the drawing, the drawing does not disappear if it fails or succeeds. If it fails, the wand poofs. If it succeeds, the spell goes on to the end of the wand… the usual. The drawing is replaced with an identical colored drawing showing tension. When the player begins drawing again, the colored-tension drawing disappears. This way a player can evaluate their successful drawings as well as their failed drawings. The player closes the book, and the feedback system is turned off.