i know exactly how you feel. this is something i've struggled with myself. from one amature to another, let me save you some headaches by sharing my discoveries with you.
misconception: paintings have no line work. this is just plain false. most painting you find will have linework in them to help define forms and edges. they're just very subtle. most often, these lines will be very thin, and be of a hue thats a bit darker then the base tone of whatever your working on. these lines will also be blended into the form slightly, usualy only keeping one side sharp, with the other gradualy fading into the subject. look around at some digital paintings, paying perticular attention to the edges of the forms. youl probably spot some of the lines when you know to look for them. also,someparts of a painting will have enough contrast to not need a line to help define it. so dont put one someplace it isnt needed. only use lines on places that need the added deffinition.
methodology: personaly, in my painting i work in grayscale first to get the values right, then add color later. so when i work lines into a painting i just use some shade of dark gray orblack, but from the looks of your sample peice you work in color from the get go. off the top of my head i would reccomend a custom brush preset for painting in your lines. perhaps opacity and flow set to 20% with brush size set to pen pressure, and blending mode at multiply. this will let you gradualy work in a line with the exact amount of contrast you need. dont take that as gospel though, exparament with it a bit and find whats comfortable for you.
also, never underestimate the importance of rim-lights. i cant really think of a real-world situation where there isnt atleast -some- ambient light bouncing around to produce a rimlight. even if its only a very dim one. rimlights are very useful for defining edges agenst backgrounds of like-color.
