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; partners for life. In Ocarina of Time, the only game they appear in, they basically just float around their child's head and follow them wherever they go. But in manga adaptions they play a more active role, like talking or pushing people over. ^^;

However, there is something about these guardian fairies that separates them from the other forest fairies like Navi, Tatl, and 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 will turn a different color when friends or foes approach them, but as soon as it has moved on, the fairies return to their natural color, blue for Navi and whitish-yellow for Tatl. The KGF on the other hand constantly change colors regardless of the situation; their aura appearing in every shade of the rainbow. Every time you see them they will be a different hue, even for the more prominent 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. Navi is also similar to them because they all have a white center with only their aura/outer-most part colored. Alternatively, in Majora's Mask Tatl and Tael are mostly a solid color, and in Phantom Hourglass Ciela, Leaf, and Neri are completely one solid color, including their wings. So perhaps the color patterns differ from region to region.

Another interesting note is that in The 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 find only in the Forest Haven. The Forest Firefly is a "mystical creature" in which "a strange light emanates from". Physically, there are an incredibly bright orb of light that is constantly changing colors. It is also much larger and brighter than the other forest sprites as seen in the photo. So it seems plausible, 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. ^_^;