ChristopherStevens' Maxx - Color

Hi, sorry to bother you, but i have a question which i hope you will be willing to answer, it would be greatly appreciated!

I was wondering, how do you color a gray scale image in Photoshop? I usually scan my line art and fill it from behind. Unfortunatly that means it's simply line art with no shadows or anything. how are you taking a fully shaded piece and adding vibrant colors to it?

yeah, sorry, but I'm pretty clueless to how you did this. I'm fairly basic in my knowledge of Photoshop and what little I have learned from youtube videos and stuff like that.