nifty, I wouldn't mind seeing more.
The top of number 5's head looks odd where the forehead meets the scalp, because the shadeing curviture becomes inversed, which makes the top look indented. The top of the head extends higher than what would be considered natural. The ear looks funky.
Number 6 has an unnaturally small noggin(fits well with his unnatural ear), and under his ear has an odd dark area(where the neck should meet). The shadeing seems to be handled better on this one. His eye-socket seems overly wide compared to the eye.
Seems like you are doing a bit of experimenting with the face features.. From the uncolored ones, i'm like the 3rd ones face most due to the simplicity compared to the others. The one on the farthest right is cute(the squarish one).