I have not much good experience with applying the ink directly to the metal, as it is very colour-intensive. You'd definitely have to thin it down.
On the other hand, I have nothing but good experience with mixing the ink into the colour. For "shiny" armour, I have a constant mix of chainmail and blue ink which looks great in my opinion. The ink improves chainmail much, it looks so much smoother. It might be a personal thing, but I find pure chainmail a bit boring. Add as much ink as you feel appropriate, though even a little amount will do wonders.
Conversely, I have tried to mix some metal colour into black ink, then add more metal subsequently until highlighting with pure boltgun metal. It makes for a very dark armour, but I haven't tested how this looks on larger pieces like bardings.
With red, I'm not so sure, as I am uncertain what you strive to achieve. Is the sword supposed to be bloodied?
But I'm by no means a professional, that's just my personal experience.