Mottling is a special effect that can appear when Blush or White Markings or Albino mutation are presents.
It causes the skin to show spots, it appears where the skin is softer (like on groin and muzzle)
The spots can be only darker than what sits under, can be of different opacity, and should be very faint.
The area affected by mottling can have cutouts/holes! So mix and match the spots and mottling as you please
In case a white marking touches the eye or mouth, the mouth or eye color can split because of mottling.
Marking idea by @Scribblewoof
In case of White Marks or albino, mottling concentrate mostly on the pinky areas
When no mark is present, mottling can show on its own, can be mixed with Blush