It’s okay, english is not my native language either but I totally get what you mean :3
Hmmm it’s a very good question. I actually had to take a look at my oldest drawings in order to find out how and where I actually started painting single-layer style.
Due to certain circumstances when I was younger, I used programs like Oekakiboards to make my pictures. And those programs have a lot of limitations.
When I couldn’t have more than 1 or 2 layers, I pushed myself and got used to have all the colors on the same layer.
Also, I went to art school in High School - where I got to try out tradition opaque-paint (oils, acrylics, guache etc) which was very fun. When I got Corel Painter, it had some limitations as well (the latest version has a lot of improvements tho) but thanks to the blender-tool in Corel, as well as my basic knowledge for traditional drawing/painting, I started with shapes and silhouettes instead of lineart. I soon learned that I actually have more control over the painting when it wasn’t “trapped in a lineart”. If I saw that the hand looks odd, I can easily re-paint it - if you have a lineart in that situation, you would have to first re-draw the lineart and then adjust the color-layers and so on.
Also, lineart doesn’t exist in reality - the closest to lineart are actually tiny shadows between folds, cracks etc.
I often start with a silhouette, and then maybe I do a rough sketch to get the proportions right. And then I just paint over the areas where you would normally have lineart.
However, one of the first techniques that probably helped me out was this one:
You start with the sketch - or lineart, it’s not so important to have the lineart perfectly cleaned since it will be removed later.
Next, on a layer below it, you add the values and colors. it is important to include light and shadow as well. At this stage, I very often reduce the opacity of the lineart-layer.
Now I delete the lineart layer, and voilá! I have a pair of lips without lineart!
This technique was the first one I used for this. Later on, I started with silhouettes and values. But I think this might be a good technique for those who really REALLY have a hard time grasping the whole concept.