I think as I inspected the borders, there are basically 0 repeats both horizontal as vertical of the texture.
So there is no real pixel displayed on the sides from the original texture while this texture is applied.
This is likely done to avoid an unwanted pattern on the sides.
I filed a ticket to ask a confirmation and it seems SL picks a random color or a mix of the colors which are used in the center of the texture.
So the end result is rather unpredictable still.
Especially if much colors are used in the center area.
I think maybe if KC would use a different texture offset vertical and make it so it picks the border color area it makes it easier to make it match.
Because I think most of the patterns and different colors within our textures take place in the center.
The closest collar has a tweaked vertical offset while the other is set to zero as it is currently offered to us in my opinion the white border doesn't blend as well as with the tweaked offset.