VerticalGradientVertexColor.py

import bpy

def paintVerteces():
    ob = bpy.context.object
    print(ob.name)
    bpy.ops.object.mode_set(mode='VERTEX_PAINT')

    maxZ = float('-inf')
    minZ = float('inf')

    for face in ob.data.polygons:
        for vert_idx in face.vertices:
            position = ob.data.vertices[vert_idx].co
            z = position.z
            if z<minZ:
                minZ = z
            if z>maxZ:
                maxZ = z

    for face in ob.data.polygons:
        for vert_idx, loop_idx in zip(face.vertices, face.loop_indices):
            position = ob.data.vertices[vert_idx].co
            z = position.z
            color = remap(minZ,0,maxZ,1,z)
            bpy.context.object.data.vertex_colors["Col"].data[loop_idx].color = (color,color,color,255)
    
    ob.data.update()
            

def remap(min1,min2,max1,max2, value):
    return min2 + (value - min1) * (max2 - min2) / (max1 - min1)

        
if __name__ == "__main__":
    paintVerteces()