Now, what got me thinking is that these eye colors can be accosiated with different seasons.
Yellow - Fall
Green - Spring
Red - Summer
The green is obvious, yellow and red less so. My first inclination is that red is the "hottest" color so it should be summer and yellow would be fall, as leaves tend to turn yellow in the fall--among other colors. However, we could also go with:
Yellow - Summer
Green - Spring
Red - Fall
The reasoning behind this combination is that the yellow eyes of the singers are described as "as yellow as the sun," and the red eyes are described as "as red as blood." Blood and red also work for fall, as things are dying--hence the blood--and also because as we all know that some leaves turn red in fall--again though, among other colors.
Now, if there is indeed a pattern--and it would make sense that there is, as the singers are very much tied to nature and its workings in the books--what happened to winter? Why is this pattern unfinished? ARE there children with blue eyes? If so what does this signify for them? Have they too been chosen by the gods just like those with red and green eyes? If so, what have they been chosen for? Could these maybe-existant blue-eyed singers have something to do with the Others?
Like I said, a little speculation on something that caught my eye as soon as I read it. Could be nothing, but it did seem to me as if there was a pattern there and the fact that it wasn't finished stood out to me.