How to set "real" color with 'MaterialRecord'

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I use the code in the #6260 :
```
#!/usr/bin/env python

import numpy as np
import open3d as o3d

RED = [1.0, 0.0, 0.0, 1.0]
GREEN = [0.0, 0.5, 0.0, 1.0] # dark green is easier to see on white when small
BLUE = [0.0, 0.0, 1.0, 1.0]

def add_cloud(scene, name, npts, center, radius, color, size):
pts = np.random.uniform(-radius, radius, size=[npts, 3]) + center
cloud = o3d.geometry.PointCloud()
cloud.points = o3d.utility.Vector3dVector(pts)

material = o3d.visualization.rendering.Material()
material.shader = "defaultUnlit"
material.base_color = color
material.point_size = size
scene.add_geometry(name, cloud, material)

def main():
app = o3d.visualization.gui.Application.instance
app.initialize()
window = app.create_window("Open3d", 1024, 768)
widget3d = o3d.visualization.gui.SceneWidget()
widget3d.scene = o3d.visualization.rendering.Open3DScene(window.renderer)
widget3d.scene.set_background_color([1.0, 1.0, 1.0, 1.0])
window.add_child(widget3d)

add_cloud(widget3d.scene, "green", 5000, [0, 0, 0], 10.0, GREEN, 2)
add_cloud(widget3d.scene, "red", 500, [-2, -2, -2], 5.0, RED, 4)
add_cloud(widget3d.scene, "blue", 250, [1, 1, 1], 5.0, BLUE, 6)

widget3d.setup_camera(60, widget3d.scene.bounding_box, [0, 0, 0])

app.run()

if __name__ == "__main__":
main()
```
First, I found the backgroud color is not "real" #FFFFFF, so I add a line to set backgroud color to "real" #FFFFFF.
```
widget3d.scene.view.set_post_processing(False)
```
Second, I found the MaterialRecord.base_color is also not "real", I set #FF0000 will get #7F0000, #00FF00 will get #007F00, #0000FF will get #0000FF. I'm confusing how to set the "real" color.