Grading Based On Red, Red Brown, Brown For Lincoln Wheat

Color should be mostly independent of the numerical grade, however you will usually see brown coins on the lower end of the spectrum than red coins, I would guess that's usually more because the conditions the red coins are stored in are better than the brown ones, resulting in less imperfections. Also luster is usually less apparent on brown coins and luster is a large factor in MS coins(especially higher grade MS coins)

All in all a brown MS coin is usually indicative of a lower MS grade, but not necessarily because of the color. You can find flawless brown coins that have insane luster and be very high grade. And conversely, you can find extremely red MS coins that are bag marked to heck and only fly as MS because there's no wear.

Reading Red Book toning of copper is natural. And while red coins will sell for more, many are doctored (dipped) to show the red color. An ms 64 red or ms 64 brown are the same condition grade but the red coin will generally sell for more money. My favorite are red/brown coins as they have both luster and original mint color