Hrm, don't quote me on this, but to me that looks like either a Specular or Shading issue. Is that a hat hair or an accessory?
Try the No Stencil mod
http://modthesims.info/d/478307 Get rid of the Specular on the accessory in CASt by typing in [Shift] + [Ctrl] + [C] and then typing this:
deco p then pressing [Enter].
If the texture fixes itself, then you need to fix your Specular texture you made for it, and make it and the alpha channel more black or more white or whatever. (Or just be a lazy wretch like me and leave it alone and direct everyone to the No Stencil Mod to apply the change in CASt themselves like I do!
)
If the texture still doesn't look right, you may need to fix the shading of the mesh itself -- in Milkshape try anything from Align Normals, to Smooth All, to Auto-Smooth, to Model Cleaner -- anything to fix the shading. For me it's always very trial and error; fiddling around until a miracle fixes everything for me.
That helmet looks SUPER SEXY though!
Don't give up on it yet, cuz it's 95% perfect.