Well Since nobody else left a comment I'll try and answer this from what I understand this is Nasu statement

Shiki's eyes were meant to "see that which cannot be seen". However, after having two near death experiences he was somehow connected to (the Root), and his brain began to be able to comprehend death itself. Or to put it in other words, it is only by virtue of his mystic eyes and brain working together as a set that he is able to perceive death.By the way, Shiki's eyes turn blue when he intentionally tries to see death. Seeing as how blue is the color of pure eyes, it's likely that Shiki's eyes were originally of that type.

So Shiki eyes where pure eyes but they got connect to the root which is why he can see death also MeoDP are a hybrid power that take aspects of mystic eyes and pure eyes

Q: Are Ryougi's Mystic Eyes of Death Perception a type of Eyes of Clarity?A: Yes. The Ryougis and Nanayas were good friends! (A long time ago)Although the two types of eyes are inheritable, if you consider regular Mystic Eyes as traits that modify the body..... something like the inheritance of the traits of the parents' body...... then Eyes of Clarity are the inheritance of fate and destiny......the offspring continues the parents' will and their quest.Even though Eyes of Clarity are the parents' karma reflected in the offspring, they can only be inherited by "humans" with high spiritual potential.In other words, Mystic Eyes of Death Perception are a type of hybrid power. The cheating powers of both Shikis ought to have a limit!

This is the reason why Tohno can see Ahika power hair thing

And we don't know what Tohno normal Pure eyes allowed him to see however we do know that Shiki father pure eyes allowed him to see thoughts in the form of colors so maybe Shiki would have done the same for him not like it matters since MEoDP are a thousand times better