OpenGL colors interpolation [closed]

I am using pyopenGL in developing a FEM application
Why am I getting this result when rendering some quads
Values and colors are symmetric
enter image description here

Some ideas for obtaining better result
here some lines of the code:
Shaders:

GL_VERT_SHADER = '''
#version 330 core
layout (location = 0) in vec3 vertPos; // Input position
layout (location = 1) in vec4 vertCol; // Input color
smooth out vec4 color;
uniform mat4 finalMatrix; // The final matrix
void main() {
gl_Position = finalMatrix * vec4(vertPos.x, vertPos.y, vertPos.z, 1.0);
color = vertCol;
}
'''

GL_FRAG_SHADER = '''
#version 330
smooth in vec4 color;
void main() {
gl_FragColor = color;
}
'''

VAO and VBO

        glBindVertexArray(self.vaomquad)
        glBindBuffer(GL_ARRAY_BUFFER,self.vbomquad)
        glBufferData(GL_ARRAY_BUFFER,data=vertquads,usage=GL_STATIC_DRAW,size=sizeof(vertquads))
        glEnableVertexAttribArray(0)
        glVertexAttribPointer(0,3,GL_FLOAT,False,28,None)
        glEnableVertexAttribArray(1)
        glVertexAttribPointer(1,4,GL_FLOAT,False,28,ctypes.c_void_p(12))
        glBindBuffer(GL_ARRAY_BUFFER,0)
        glBindVertexArray(0)

Drawing:

            glBindVertexArray(self.vaomquad)
            glDrawArrays(GL_QUADS,0,self.nquads*4)
            glBindVertexArray(0)

For the colors interpolation I used this function:

    def interpolatecolor(self,v,vmin,vmax):
        anum = 1020/(vmax-vmin)
        bnum = -anum*vmin
        numcol = v*anum+bnum
        r,g,b = 0,0,0
        if (numcol>=0 and numcol<255):
            r = 255
            g = numcol
        elif (numcol>=255 and numcol<510):
            r = 255-(numcol-255)
            g = 255
        elif (numcol>=510 and numcol<765):
            b = numcol-510
            g = 255
        elif (numcol>=765 and numcol<=1020):
            g = 255-(numcol-765)
            b = 255
        return (r,g,b,1)

I resolve the problem by adding the barycenter of the quad so that I had to render a quad primitive as a group of four triangles (12 vertices per quad). Interpolation is now based on the four vertices of the quad.
enter image description here