Unable to set color to pointcloud constructed from read_point_cloud()

[NNrico]

Hello,
I cannot color a pointcloud loaded with read_point_cloud() function. The problem is shown by code below with open3d version 0.19 and python 3.12

import open3d as o3d

path_pcd = r"few points.ply"

pcd = o3d.io.read_point_cloud(path_pcd) 
pcd.paint_uniform_color([1, 0, 0])
o3d.visualization.draw_geometries([pcd])

I am loading a pointcloud , color it in red and display it, but the color is black but the array is of red colors

If I try this workaroud it works

import open3d as o3d

path_pcd = r"few points.ply"

pcd_from_file = o3d.io.read_point_cloud(path_pcd) 
pcd = o3d.geometry.PointCloud() # create new pointcloud
pcd.points = o3d.utility.Vector3dVector(pcd_from_file.points) # copy the points
pcd.paint_uniform_color([1, 0, 0])
o3d.visualization.draw_geometries([pcd])``

The workaround consist of creating a new point cloud and copying to it the points from the pointcloud loaded from file. I feel I should not do this to color a popintcloud loaded from file.

Is this a bug?

[lee-deraud]

Is there any color data present in the actual .PLY file?

[NNrico]

Right, I should share the file "few points.ply".

Below I share a lighter pointcloud

ply
format ascii 1.0
comment VCGLIB generated
element vertex 9
property float x
property float y
property float z
property float nx
property float ny
property float nz
property uchar red
property uchar green
property uchar blue
property uchar alpha
element face 0
property list uchar int vertex_indices
end_header
636.3973 590.8067 1648.21 0 0 0 0 0 254 0 
636.3975 592.3282 1648.661 0 0 0 0 0 254 0 
636.296 593.8759 1649.183 0 0 0 0 0 241 0 
637.1255 581.217 1648.002 0 0 0 0 0 254 0 
637.1493 582.5253 1647.858 0 0 0 0 0 254 0 
637.0212 583.8083 1647.643 0 0 0 0 0 254 0 
636.959 585.2737 1647.943 0 0 0 0 0 254 0 
636.9749 586.7282 1648.21 0 0 0 0 0 254 0 
637.0667 587.9985 1647.96 0 0 0 0 0 254 0 

When I load the file i have this values in the colors field

[lee-deraud]

Those zero alpha values are suspicious to me, even if they're unused by the open3d pointcloud object.
I'd be tempted to edit them out of the smaller file (along with the 'property' line for them) and see if the symptoms change.
(I'd do it but I'm at the wrong computer at the moment...)

[NNrico]

After playing with the values I found the reason might be the fact that in my point cloud the nx ny nz values are set to 0 0 0 if I set the normal to a non zero vector the color changes correctly as expected.