It sometimes goes red too, what does it mean?

I don’t know you tell me

Glad you asked! That (R) is the indicator of updates to the world model the game uses for performance, when it flashes red that means the world model has updated and had to rebuild in the same frame. This can happen if a brick is created or a simple brick converts into a fast brick because that brick was updated (such as changing colors).
https://www.brick-hill.com/forum/thread/1453470/
https://www.brick-hill.com/forum/thread/1460160/