Exactly how does SMP fade?

Something that I'm worrying about is getting the "solid ink" look in the future. I've seen a lot of pictures of people who had had the treatment and noticed that after a while the individual pigments start to kind of fade together into what looks like a solid slab of ink. I'm worried that down the road I'm going to have that look in the front of my head (where I'm getting the treatment) while the sides and the back continue to look natural...