Ah OK, so it’s about ROIs. It is different from what I was investigating. FYI this is what interaction handle means without the special context you just mentioned:
The code is quite similar for ROIs though:
I think customizing the color could be solved by adding members for the interaction handle colors to the markup display node (initialized with the hard-coded colors), and using these members in the two functions I have linked about colors. Note that in ROIs you have not three but six interaction colors, so this would need to be handled somehow if you want the left-right colors to be different (note the same red color setting for L and R). Maybe we could have X, Y, and Z interaction handle color getter/setters, with optional AlternateX or something similar for the inverse directions in the ROIs. @lassoan what do you think?