Cent Grading "Red"

There is no official definition of Red "RD". It's odd because full mint luster is more orange than red but RD is what is used to define "as minted" color for modern coppers.

Copper is very reactive and quickly begins turning brown "BN" if exposed to the environment. Hence Red-Brown "RB" for those strarting to age not so gracefully.

Most older, circulated coppers are brown "BN". I find brown cents to be more attractive than "RB" but that's just me.

the "definition" that is used by the legitimate grading companies is..

RD refers to 95-100% of mint luster still present
RB refers to 5-95%
BN refers to 0-5%

"brown" actually means any color other than untoned orange

by not requiring exactly 100% of the luster to remain, the grading standards allow very minor discoloration, for example, a few small isolated spots of toning on an otherwise new-looking coin.
brown cents may or may not have some luster, but limited to protected areas around letters or in between details of the devices. the fields are usually completely without luster.
red-brown is simply in between red and brown

And all those red cents sold at a premium will turn brown. It's just a matter of how long it takes.

It depends on whether it actually is an airtight holder.

Make that original color instead of mint luster and I will agree with you. Luster and the color have nothing to do with each other and you can have brown coins that still have full cartwheel luster.