Because the objects are created with maths formulas and they are not modelt, the face orientation in Blender could be "wrong", means the face orientation is not in the way you would like to have them. XYZ Axis can be shown in the negative way and cause this problem. 

There is a checkbox to flip the faces from the geometry modifier.

But in some cases even this don´t work because a object has xyz positive & negative direction. (watch the video) so neither solution one nor two help further and you have to flip them manually. If it is needed 😃

The Materials are configured with the "set material Node" in the Geometry Node Modifier and not with the material slot. If you work with new materials, you have to change in the "Material" Box in the Geometry Node Modifier!

The first 2 sets contain "famous" algebraic surfaces, named often after the person who created them. The third set contains mostly free developed surfaces so i didn´t give a name

Prospec Regular

Each set contains a "Gradial" Material Shader, and a Basic Colour for the Grid

You have to set a "realize instance" node at the last position of the Geometry Nodes before you apply the Modifier

After applying the Geo-Node Modifier, you should use the function "merge by distance" to be sure there are no overlapping vertices (or use the Node "merge by Distance" inside the Node Tree)