Is there a way to set toolbar image/color from scripting?
The purpose of it would be for example setting reddish toolbar background for folders where image manipulation should be made with care or when a document is detected in a folder that should be acted upon.
Another useful variation could be not changing the image, but just adding semi-transparent color overlay on toolbar background.
I don't think there is a way to do that. You could have a red toolbar which you switch to when in certain folders, but you'd need a second copy of the toolbar (if you wanted a non-red version the rest of the time, that is).
You could also have one or more buttons with red backgrounds which appear when in certain folders, for a similar effect. (And similarly have buttons with normal background which are hidden in the same folders.)
Or you could change the background color of the folders.