So De Atramentis Document Ink is:
- extremely waterproof
- permanent and archival
- available in a huge range of colours including grey
- mixable and dilute-able without impairing functionality (you can buy the clear medium fluid)
- very fountain pen friendly

The only downside appears to be the price, but the per-drawing cost is so low that it seems to be completely irrelevant compared to paper and paint costs.

Have I missed something?