Procedural Lips advice

hi all i need a great help as how can i define the “existence field” of the lips on my character procedurally; since i’m very bad at drawing i was hoping i cna skip the texturing via image/paint by using math, i was able to make the eyes and skin and also define some circular body parts, but when it comes to the lips i don’t know how to keep going and adding a function that wil ldescribe the curve of the lips (i have it something that can describe it very nicely) but i dont’ know how to put in blender and show it, also i’d like that a linear gradient is applied all over the curve