Andre said:
If only a specific layer, then a reasonable interpretation of the orb spell is that there are two distinct effects, only one of which is affected by Mettle.
The sickening aspect of Orb of Acid doesn't have a lesser effect on a successful save; it has
no effect on a successful save.
But we know that Orb of Acid is an attack that has a lesser effect on a successful save, because Mettle states that any spell that has a saving throw entry of Fortitude Partial is an attack that has a lesser effect on a successful save, and Orb of Acid is a spell that has a saving throw entry of Fortitude Partial.
Since the sickening aspect doesn't have a lesser effect on a successful save, but Orb of Acid is an attack that has a lesser effect on a successful save, it can't be the sickening aspect taken in isolation to which Mettle is referring; it must be Orb of Acid. So if the save is successful, Orb of Acid is negated.
Prismatic Wall doesn't have a saving throw entry of Fortitude Partial; it has a saving throw entry of See Text. So Prismatic Wall is not, automatically, an attack that has a lesser effect on a successful save. Mettle would negate the Con damage from the Green wall, but it would not necessarily have an effect on the rest of the wall.
The Fortitude Partial save doesn't apply to the sickening aspect of Orb of Acid; if it did, it would be Fortitude Negates! The Fortitude Partial save applies to Orb of Acid.
The Fortitude Partial save doesn't apply to Prismatic Wall; instead, it applies to the Green layer of Prismatic Wall.
-Hyp.