Here is the demo - Babylon.js - Multi views demo
If I set any scene.clearColor in the code, all canvas blink when reloading the page and then show again default color.
That’s the scene.createDefaultEnvironment(); call which is creating a box, so what you see is not the clear color but the box color. If you comment this line you will see that the clear color is correctly taken into account.
you can also tweak the default environment main color 