Omg dear this is a really difficult thing you are asking to me because I’m not a lips expert ;w; I’ll do my best!
I even did gifs OHOH it’s the first time, I hope they explains better than my writing.
Lips are not ‘round’ as simpsons one, quoting you. They have more like a smoothed triangular aspect, like a cone with a round tip.
Of course it depends the style you are using when drawing lips, most anime-manga/cartoonish styles uses just a line for the lips and I do use a more complicated but still simple style for the lips I draw.
For example, I do draw the lower part of the cupid’s bow, I don’t think it has a proper name, so I’m showing you:
When I’m drawing and coloring males, I don’t color the lips because my style isn’t realistic so I don’t bother. That’s why the cupid’s bow isn’t showing (and in non-realistic styles I find it really annoying if it’s only drew, like in certain mangas, for example)
Time to show you how I draw lips.
First example, the normal smiling side view lips
As you can see the lips are little curves and it’s not necessary that the side lips have to touch the line. Also, look at the third arrow, the line between the lips is more curved because of the under cupid’s bow.
An opened mouth is different, since the inside of the mouth or the teeth divide the lips:
The smiling mouth:
Normal closed mouth, no smiling:
I am sorry, I hope it helps ;__; I didn’t even know where to start explaining! Anyway you can try to search on deviantart for more accura tutorials ;v;