Vertex Color error

Hey I have this code:

                     byte currentLight = World.instance.GetLightData(worldPos + new Vector2(0,0));
                     byte topLight = World.instance.GetLightData(worldPos + new Vector2(0, 1));
                     byte topLeftLight = World.instance.GetLightData(worldPos + new Vector2(-1, 1));
                     byte topRightLight = World.instance.GetLightData(worldPos + new Vector2(1, 1));
                     byte bottomLight = World.instance.GetLightData(worldPos + new Vector2(0, -1));
                     byte bottomLeftLight = World.instance.GetLightData(worldPos + new Vector2(-1, -1));
                     byte bottomRightLight = World.instance.GetLightData(worldPos + new Vector2(1, -1));
                     byte leftLight = World.instance.GetLightData(worldPos + new Vector2(-1, 0));
                     byte rightLight = World.instance.GetLightData(worldPos + new Vector2(1, 0));
 
 
                     // Bottom-Left Vertex
                     byte vertexColor = (byte)((leftLight + currentLight + bottomLight + bottomLeftLight) / 4);
                     terrainVertexColors.Add(new Color32(vertexColor, vertexColor, vertexColor, 0));
 
                     // Top-Left Vertex
                     vertexColor = (byte)((topLeftLight + topLight + currentLight + leftLight) / 4);
                     terrainVertexColors.Add(new Color32(vertexColor, vertexColor, vertexColor, 0));
 
                     // Top-Right Vertex
                     vertexColor = (byte)((topLight + topRightLight + rightLight + currentLight) / 4);
                     terrainVertexColors.Add(new Color32(vertexColor, vertexColor, vertexColor, 0));
 
                     // Bottom-Right Vertex
                     vertexColor = (byte)((currentLight + rightLight + bottomRightLight + bottomLight) / 4);
                     terrainVertexColors.Add(new Color32(vertexColor, vertexColor, vertexColor, 0));

And this is the result:

I found this, scroll down to "Details regarding meshing".

But I still don't know how to fix this. Does anyone have an idea?

Thanks!