I hate the idea of the trust system. Anything that is designed as a solo mechanic for an MMO is ridiculous when they should be focusing of mechanics that encourage and support party play. But as long as it's here, I'd consider using it if it could help me weakness target 100% in abyssea.
Ideally, with 3 trust slots have the player choose the job they want then have 1 trust NPC for yellow, blue and red.
If that's too overpowered, how about a set of 3 for each color that still allows the player to choose their own job.
As a final desperate plea, any combination of 3 NPCs + Player job that would make a 100% chance of targeting one color of weakness.