I did a quick build to see if I would get the same thing happening in my test game. And yes, I had the same weirdness happen with the both the fences and the spandrels. I tried to replicate that part of your build and had a lot of trouble with it. During the process, I got a lot of error messages about the round room sections and then couldn't delete the extra walls. It gave me so much trouble, that I couldn't replicate what I see in your pics. So, I tried it again using platforms and just lowered them. That worked. All through the process though, I ran into one thing after the other and finally got something that looked close to what you have. One thing I did find during the process is that if you delete the ceilings, then everything looks the right color. So, I tried adding another floor and making it a three-floor tall atrium. That didn't work and the fences and spandrels looked different colors. Only deleting the ceiling altogether made everything look right which doesn't work since no ceiling lets the weather in.
In the end, I have to conclude that the problem here is with how the lighting works with curved walls indoors. Round room sections and rounded platforms act the same way, so you run into the issue using either. If there is no ceiling, colors show as they should but if there is a ceiling present that blocks the daylight, then the colors aren't showing up correctly. So, in my opinion, this is a bug in the build system, and I think the main culprit is the curved walls and how indoor lighting affects them as it doesn't happen when you use straight walls, nor does it happen when you use the curved sections outdoors. But it should not happen and the devs need to address the issue. In the short term, the only suggestion I can think of for you is to get rid of the round sections and replace them with square sections where the walls are straight. When done that way, everything looks correct color wise.
Hope this helps.
Thank you for testing it for me! Also, I know right, I had so many problems even getting the curved fences and things to the way it is in the photos. I also noticed that the color goes back to normal when the ceiling is removed. Weirdly enough though, for a brief moment I had the same problem with some regular columns(not near and curves), until I just rotated them a few times and it went away. I'll switch my walls to straight ones, thank you for the idea!!!
