coloring help for a friend.

i wouldn't be posting here if i didn't absolutely need help but im at a bit of a coloring crisis. my friend asked me the other day how to do coloring like this:

made by toastandtea

and i really want to help her (she would post but she's kinda shy and hates asking for help...so im doing it for her), not to mention figure it out for myself.

we thought maybe color burn but i dont know.... we tried it and it didn't look right.

again, please help.