I don't know if the artist will ever answer but I think this is pretty easy...It's just a vector lineart in a different color, then I think the artist put some texture on that lineart. Hope this helps!
Ah sorry! Well do you know that pen tool in photoshop? That pen tool creates smooth even lines unlike doing lineart by yourself on tablet. I may be wrong about the lineart thing so either the artist made a colored lineart around the characters or he made a border for them (which is most likely what he did and which you can also do on photoshop). I hope you get me. xD
There are many tutorials on the pen tool and coloring here on deviantart and on the internet.
here's a pen tool tutorial for photoshop if you would like to try and learn