My Skins have a Birth Mark

Hi, I was wondering if anyone can help me, I am making my owns skins and have the templates, Everything is going well, But when I upload them and apply them to my avatar my skins have an ugly brown mark on the neck, like a seam. I have tried pretty much everything to get rid of it - Does anyone have any suggestions as to what this might be and how to be rid of it?
Thank you so much in advance
Kind regards,
Charlie

That ugly neck line comes from the mouth image being too large in the head texture. Shrink it a bit, nibbling away at it by replacing small parts with your skin texture for the neck--a trial and error process--and that will get rid of the shadowy seam at the back of the neck. The bottom edge of the mouth is fine, it's the left and right sides and the top of the mouth portion of the head texture that needs to shrink. If you are using the standard templates that float around, you may want to do some dental work, too, since you are in this area. For my taste, the standard dentition is gap toothed and a little too gray.
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Another thing that can cause this is that there's a spot on the template where part of the neck seam and part of the scalp seam are very close to each other. If the top of the head is a different color than the neck (like if you're doing a hair base), the hair color can bleed onto the neck around the adams apple. It usually only shows when the skin isn't fully rezzed, and unfortunately there's not much you can do about it except to shrink the bleed area in those spots.