Would the indigo layer of a prismatic wall spell (in a spherical shape) block the effect of a fireball, if it had radius bigger than the radius of the wall, cast outside the wall but centered/targeted inside it?
The fireball is blocked
There's a couple things regarding prismatic wall (in sphere shape) that blocks fireball in your situation.
- First, the spell cannot target a creature on the inside of the sphere because the caster does not have line of effect (nor line of sight) to that target. So were you to attempt to target someone inside of the sphere, the spell would detonate outside of the sphere.
- Second, even if the spell were set to detonate outside the sphere, it would not be able to penetrate through the wall because the indigo layer specifically blocks the effect. It is worth noting that if the Prismatic Wall spell was cast in its vertical wall form and the fireball targeted near a corner, it would wrap around the edge and potentially hit someone on the other side.