splitting hairs

The Rambam paskens (Ishus 2:16) with respect to the halacha of the two hairs that define gadlus:

שתי שערות האמורות בבן ובבת, ובכל מקום--שיעורן כדי לכוף ראשן לעיקרן. ומשיצמחו ויהיו יכולות להינטל בפי הזוג, עד שיגיעו לכוף ראשן לעיקרן--דנין בהן להחמיר, בכל מקום: לפיכך בבן ובבת, נחשוב אותם גדולים להחמיר, הואיל וצמחו כדי להינטל בפי הזוג; ונחשוב אותם קטנים להחמיר, הואיל ולא הגיעו לכוף ראשן לעיקרן.

There are different opinions of Tanaim as to the minimum length which defines a hair. The Rambam writes that we adopt the chumros of all views. So long as the hair is long enough to be cut, we treat the child as a gadol l'hachmir. However, until the hairs are long enough to bend over back to their root, we do not allow for any of the leniencies of being a gadol.

Yet, the Rambam writes with respect to Parah Adumah (1:4):

היו בה שתי שערות עיקרן מאדים וראשן משחיר, עיקרן משחיר וראשן מאדים--הכול הולך אחר העיקר; וגוזז במספריים את ראשן המשחיר, ואינו חושש משום גיזה בקודשים--שאין כוונתו לגזוז.

וצריך שיישאר מן המאדים, כדי שתינטל בזוג--שכל שערה שאינה ניטלת בזוג, הרי היא כאילו אינה; לפיכך אם היו בה שתי שערות לבנות או שחורות, שאינן נלקטין בזוג--הרי זו כשרה.

So long as the root of the hair of a cow is red, the hair is treated as red even if the top is black. How much of the hair has to be red to call it a red hair? The Rambam writes so long as it is long enough to be cut.

The Kesef Mishneh asks: Since the Rambam follows the stringencies of all views when it comes to defining a hair, shouldn't the Rambam pasken that until the hair is long enough to fold over back to its root, it is not called a red hair? Why is the Rambam lenient here and allow a hair to count as red even if it is just long enough to be cut?

R' Chaim Brisker has an answer to this question, but I would rather share R' Shach's answer (Avi Ezri, Hil Ishus) which is similar, but slightly easier to understand and a lot easier to write over : ) The difference between the two cases is that when it comes to defining adulthood, the presence or absence of hair is the key criteria; the Rambam in that case defines a precise strict shiur of what defines hair. Not so when it comes to parah adumah. There, the key criteria is redness. The presence or absence of hair is just a means of measuring the presence or absence of redness, but is not a defining feature in and of itself. A hair long enough to be cut but not long enough to bend back over may be a hair of insufficient length to define gadlus, but that does not mean it is anything other than a hair and its color is anything other than red.