I doesnít matter if you think Snape is really reformed or not, the presence of that tattoo canít be a good thing for him.
Originally, I assumed that this was Snape first noticing the reappearance of the mark, but then I though it would be cool if this was when VolÖHe-Who-Must-Not-Be-Named
summoned all the Death Eaterís to him at the end of 4th year.
Yes, I know that the tattoo never glowed green, but come on it looks cool, plus I wanted some practice with color shading (i.e. on his face). I wouldnít have been able to do that if I made the thing black, and I couldnít really see the red glowing.
Snape © Rowling
Snapeís Lion Design © Kira617