I already reported this bug in another thread as a reply to a totally different bug, but I thought that it would receive a better recognition if it has a dedicated topic:
The latest build of Rhino 9 Beta will ignore the object colour and will only switch to the proper coloured object rendering mode when the control points are turned on. This particular bug was introduced recently, so my hope is that the developers could figure out what causes it easily and fix it in the next build of Rhino 9 Beta.