I had an occasion to review a Venetian Plaster installation on a two story interior wall, one side radiused and the other side flat. This process is difficult to spread well and just as difficult to judge.
The radiused side had several open areas horizontally oriented where the finish had dried lighter than the overall color. In the picture below, you see that long consistent lighter color at a regular height on the wall.
The visible line is not a cold joint, but a scaffold line on the wall.
Inconsistent or different plasterers or tired plasterers doing the same thing with the same material produce different finished walls.
The other condition to be aware of is the light source. Lighting here is highlighting the differences. More direct lighting could have minimized them.
Mostly understand that having a customer who finds your work pleasing to the eye is critical to your success. I suggested to my client that the job could be aesthetically acceptable but according to Plastering Industry Standards it was slightly outside our minimums.
If you have questions about this condition or other situations, call or email: