I have never correlated what you are saying - nitrogen deficiency is based on color of corn facing away vs facing towards corn. When I look at a field of corn, and see differences in color looking towards vs looking away, I try to find an angle to take that variation out of my point of view.

However, I think your thought process is on the right track. Nitrogen sufficiency or in sufficiency is related to the color of corn.  A Chlorophyll meter or a GreenSeeker will measure the amount of light that reflects from the corn leaf, and is dependent on the amount of chlorophyll inside the leaf..... and that is calibrated back to sufficiency or insufficiency levels of nitrogen in the corn.

I will also say that the color of corn can - will - vary according to the genetics of that variety. So, you could have two fields of corn side-by-side, one is slightly darker green than the other one, and both will have sufficient nitrogen. 

I use the terms "sufficiency" and "insufficiency" as opposed to "deficient" or "surplus". There is such a thing as hidden hunger of nutrients.  I believe sufficiency and insufficiency are better terms for what you are describing.  To me, when you talk about "deficient", you will have visual nitrogen deficiency symptoms showing on the plant - firing of the bottom leaves, etc.  That is my perspective.