As requested here's how I managed to do the tat on my Catachan.

Started with a thin wash of blue ink, glaze medium and water, ratio would have been about .25:1:1.
Did a second coat while the first was still wet, and found that it was still way too faint so I dipped thip of the brush in some blue paint and mixed that into the wash as well.
After I had the shape sorted, I tidied it up with the flesh tone directly under it.
And thats it, after it dried out properly it actually looked pretty good.
Give's the look of a tat thats been there for a while, or a cheap in the field job.