Colouring b+w images/icon posts

Firstly, I was wondering how people colour in black and white images, say like comics or manga. For instance, in this icon by milkfed.

My second question is about layout... how do people set their icons out so neatly in icon challenge communities and such? They're kind of in boxes in rows... and stuff... I don't have an example, I'm sorry. :\

I'd appreciate it if anyone could answer either/both of these questions, as I'd like to know before I start an icon challenge of my own.