Define Red Cent For Me Please

A coin with full, original mint luster. The way a cent coin looks right after coming out of the mint.

I guess I've seen red cents then...

How can one estimate 90-95% full color? Are there certain characteristics that help you grade a cent as "red"? or in the alternative, are there characteristics that make a coin not red anymore? what is the threshold line?

That is where it tends to get subjective and it is a matter of experience at that point. To complicate matters even further, a coin once slabbed as RD could even become a RB over time since slabs are not completely airtight and storage conditions matter a great deal. Over time, copper will naturally oxidize to a more mellow brown color. The colors are generally only used for MS coins since a circulated coin will turn brown once it has floated around in commerce a bit. Uncirculated brown copper can still have luster, but what matters most is the color. A general guideline is RD is 95-100% red, RB is 95-5% red, and BN is 5-0% red. As you can see, RB has a huge range in percentage and because of that, they also tend to have a huge range in eye appeal. Obviously, a 90% red coin in the RB category should be much nicer than a 10% red coin in the RB category and there will definitely be a price difference as well skewed towards the higher percentages of red.