Don't worry.
This looks like a bug (or a feature?) of the API in SU6 and SU7. I never found any workaround.
Why do you think the divider in Stretch happens to be black!?
However, in 2D mode (view.draw2d), you can control the color.
Henry, one thing you always need to keep in mind; layers don't separate geometry in SU at all - they just control visibility. Therefore if you have basic ("primitive", "raw" - i.e. ungrouped) geometry within your model, you shouldn't put different parts of it on different layers because these parts - though on different (sometimes hidden) layers - still interact with each other. See this video for basic ideas and mistakes to avoid: