On pokemon except shadow types
NOTE: this is in Pokemon Colosseum and Pokemon XD, where there is a SHADOW type (it's not dark or ghost)
According to bulbapedia:
In Colosseum, Shadow moves are regularly effective against every other type.
In XD, they are super-effective against non-Shadow Pokemon and not very effective against Shadow Pokemon.
I'm almost positive that it's a x2 weakness on regular Pokemon, and a x4 resistance on Shadow Pokemon. They made the resistance so large to balance out the overpowered nature of Shadow Pokemon.