You can change the color of the mesh by changing the color of the material(which I assume is
SpatialMaterial and is in the material_override property). So to change the material's color, you run this code.
"Your MeshInstance".material_override.albedo_color = "Your new color"
But here is a thing to note. If multiple mesh instances share the same mesh/material then all of them will change if you try and change said mesh/material. So if you have, for example, two mesh instances with the same material, changing the material's properties will change for both of them. So to change the individual color of each object, they will each need to have a unique material.