Auto Vertex Painter is for Coloring as fast as possibile the vertex in your mesh, in Godot Engine, you can fill the vertex color or you can also use interpolation base on the vertex position or normal to fill the color of your meshes
Youtube Tutorial: https://www.youtube.com/watch?v=kgl3EnQCU0A
After activating this plugin, Each time you select a MeshInstance3d with a Mesh a V Paint
button will apear in toolbar, and if you click on that you can change the vertex color uniformly or you can interpolate between Color A and Color B base on their position or normal
Please Note after changing the vertex color of your mesh a new mesh will be created so if you saved the original mesh in resource directory it will be not changed until you save the new mesh on that, OR you can keep the Original mesh too