Unity Mesh Rendering Issue

Here is an issue I am having with my procedural generated mesh. Initially the mesh displays perfectly which I have tested with a default diffuse material, all normal's point the correct way forming the cuboid shape.

However, when I start altering vertex colors this happens.

I really can't figure out whats going on, I initially thought the issue lay with normal recalculation as the cuboid displays perfectly when looking from the inside out but neither flipping the triangles array or recalculating normal's seem to fix this issue.

Any help would be appreciated... There is a lot of code for this project so I am happy to share as needed.