Yellow turns green in sunlight.

[Jester1167]

Always proof colors in the intended environment and if you are going to laminate it, make sure you are proofing it with the laminate. Some laminates can shift colors towards yellow. On big jobs, you can even have your customer come by and sign off on the printed proof for CYA.

Also, know that colors will look different on a cloudy vs. sunny day. If your greys have any hint of green on a sunny day, they will definitely look green on a cloudy day.