It is said that every Kokiri has his or her own personal fairy, appointed to them by the Great Deku Tree. These fairies are to watch over and guide their Kokiri child, supposedly from birth to death. They are partners for life. In OoT, the only game they appear in, they basically just float around their child's head and follow them whevever they go. But in the manga, they do much more, including pushing people over. XD

However, there is something about these guardian fairies that separates them from the other forest fairies like Navi and Tatl/Tael. The Kokiri Guardian Fairies (or KGF for short) never stay the same color, unlike the other fairies who have a "normal-setting". Example:

  
Navi     KGF

Let me explain better. Navi and Tatl completely change colors when friends or foes approach, but as soon as it has moved on, they return to their natural color, blue for Navi and white for Tatl. The KGF are constantly changing colors though, their aura appearing in every shade of the rainbow. Every time you see them they will be a different hue, even for the more important Kokiri like Saria and Mido.

I took some pictures of Mido from my OoT game, and as you can see from the screenshots below, every time I saw Mido then left the room and returned, his fairy was a different color.

What does this mean? That the KGF are an entirely different breed of forest fairies, perhaps only born when a Kokiri is born. But besides that, maybe the KGF change colors to amuse their child, or reflect their current mood. Or maybe they're iridescent like hummingbirds and change colors depending on how the light hits them. It could also reflect age; maybe when a fairy is young they can change their color at will, but as they grow older it slows until they finally stay at one color. But who can say how old they are when they guide immortal children?

Whatever the reason, the KGF are certainly a unique breed of fairies, perhaps only found in Kokiri Forest. However Navi is similar to them because they all have a white center with only their aura/outer-most part is colored. Where as Tatl and Tael are entirely certain colors. Perhaps the color patterns for fairies differ from region to region.

Another interesting thing is that in Wind Waker (which takes place a couple centuries after Ocarina of Time), the Kokiri children have changed into the tree-like Koroks, and no longer have their guardian fairies. What became of their partners, I wonder? If the Kokiri changed, maybe their fairies did too. My theory is that they became the Forest Fireflies that you can only find in the Forest Haven. The Forest Firefly is a "mystical creature" in which "a strange light emanates from". IE, a tiny orb of light that is constantly changing colors. It is also much larger and brighter than the other forest sprites as seen in the pic. So it makes sense, since Forest Haven is where Kokiri Forest use to be, and they both have the same color-changing behavior. Because they are referred to as fireflies instead of fairies, maybe they lost their magical powers and that's why they don't guide the Koroks. But at least they can still help aspiring pictographers. ^_^;