I'm using your base nutrients (Grow, Bloom. Micro) and Frost and growing in Coco and having some difficulties with Nitrogen. My pH is at 5.8 and I'm feeding as per your schedule, What should I do?

A couple of important things are pH which for coir/coco should be 5.8 - 6.3 so you are in the correct range but might want to raise it a touch perhaps to around 6.0. 
Coco usually requires a bit more calcium so maybe add a bit of Monster Cal (weekly) because it has (N) nitrogen, calcium, magnesium and iron and that should improve the color and the nitrogen level. The other thing you could try is adding a small amount of N-27 until the color improves.
Look at the newer growth not the older growth down below as it doesn't matter as much or indicate the health of the plants as much as the new leaves.