You can only remark a white to another color in a cable (never in a raceway) for specific applications.
I can not tell you we have never used 12/3 for IGs but you better be sure the inspector is OK with this, changing it after is tough.
The cheapest code compliant option that I know is to use 12/2 AC hospital grade.
The metal armor can be the 'normal' ground and the insulated green conductor can be the IG.
Another option is 12/2 IG MC which has two insulated green conductors one having a yellow stripe for the IG.
I think the 12/2 IG MC is more $ than the 12/2 AC hospital grade.
