Welding vertices lightened texture colour

The picture above is the original object. It is a photogrammetry mesh generated by a computer and is the only object in the scene.

I tried to clean this mesh by using the welding modifier in Blender 2.8. When in Object Mode, in Modifier Properties, I chose Add Modifier → Weld → 0.001 m as Distance and 1 as Duplicate Limit → Apply.

The result is that I get some whitened texture, looks like the street lights are on.

Is it related to the normals?

Maybe therr are custom normals? Try to clear them and/or recalulate normals.

I did recalculated normals (Shift +N) in edit mode, but it just made more places in the model look white. What do you mean by clear them?

If the mesh has some, it could make the mergeing lokking werid. i mean Object Data Properties → Geometry Data → and then some like clear custom normals…

I tried your method, but it removed something. However, I observed something strange:

I add the normal modifier without applying it, the dark areas shown above quickly turned bright. The road now has the same bright colour, so everything looks great.

So, maybe the bright ones are correct, the dark ones are wrong. But what causes that?