One way to make the mouth solid, would be to create a new layer beneath the mouth parts containing a dark tone, like black or dark grey. This colored area would need to be the exact shape of the mouth, so that it would only cover the camo in that exact area.
Creating the exact same shape is easy. I will often just make a duplicate of the layer, in this case the mouth marking. Place this layer directly below the original. Now, for the newly copied layer, set the 'preserve opacity' to ON. Then just brush over the mouth on the new layer with the airbrush in a dark tone. The paint should perfectly flow into the needed shape. Be sure that this new layer is 100% opaque, to hide the layers beneath it.
With this layer between your mouth marking and the camo, you should get a better solid effect. I hope that's clearly explained, it's easier to 'do' that 'type'.