My INSTAR paint line is going to be offering different colour metallics other than golds, bronzes, silvers etc so there is more uniqueness with them as well as better for highlighting other metallic colours the same way you would highlight a non metallic colour.
However, adding ink would take away the metallic-ness (is that a word????) Because it would begin to water it down too much. During early experiments when we were starting to develop our metallic paints, we tried using. a metallic additive and then adding colour to it, but this failed miserably and didn't really give the desired effect we wanted. We even tried silver metallic paint and normal paint together in a 50/50 mix, but this basically ended up not being metallic at all, all that happened was that the colour if the paint ended up brighter but with no metallic presence, at least none that could be seen, so it would stand to reason that your idea would probably end up the same way.
Ultimately, metallics are not easy to make even though they may look it. This also reflects in the price difference of metallics as they take extra work to get them to a suitable level for model painting.
As
AllSeeingSkink said, you'd be better off finding companies that make off the shelf colours, you could try airfix or humbrol, by even then, they are pretty limited as well.
Metallics are not for the faint hearted, paint is one thing, metallic are quite another!