How to fade in/out whole scene

i have seen some questions about fade in/out one texture, but in my scene, there are some GUI elements and 3d scene together,first, i want to change the RenderSettings.ambientLight, but it can only change the 3d scene to black, then how can i fade in/out whole scene to black/white, thanks

Create a GUI texture that covers the entire camera then ajust the transparancy of the texture to fade in and out