You need to build a 3 dimensional gamut for color segmentation. The problem is that the gamut of skin, if you're going to consider all skin tones for all races, is not a shape that can be easily thresholded in any color space. It's a banana or boomerang shape which is a strange shape to carve out - you can't use a high and low threshold on color space planes and be perfectly accurate, because then you'd be saying gray things in the image are skin. But you need neutral colored regions being called skin for some cases like very pale people or very dark people, but just not L or V values that are in between. However if you know you don't have grays you might be able to get something that works. See attached demos.