I am wondering about whether to get Linux certification, and how valuable it would be. Not valuable as "marketable" but valuable personally.

Anyway, I wonder if it would differ from MS certification in the way that really counts - would it be "grey" knowledge or "green" knowledge?

I would call "grey" any knowledge that is arbitrary facts you just have to remember. Eg. which menu option to choose to do a word count, what year a certain battle took place, how to conjugate an irregular verb. The facts may be useful as they are, but that's all they are.

"Green" knowledge reflects an underlying reality, and can grow by interlinking with other things, related or otherwise, to contribute to a deeper understanding. Not just how things are, but WHY. E.g. how to use pipes to filter text files (including getting a word count), what influences caused a battle to take place, how conjugation of regular verbs works and which verbs to expect to be irregular.

So, would Linux certification leave me with a better mental map of OS design and good practice, or just a head full of "how to install Red Hat"?