I was wondering if there's a way to update the default window color when an image is buffering. Currently it's white.
For example, if I have a workspace open with just vscode, when I switch between workspaces there's a brief moment when the screen is white and I'd prefer the monitor be painted black while the image is buffering.
Sometime this issue also happens when moving windows around but it covers a smaller portion of the screen.
There is a setting that may be precisely the answer to your question: change the buffer color to black (Check the section in the manual) Particularly, for i3 version 4.21, the config would be this:
client.background #000000
If that does not solve the issue, perhaps you could address it from a different perspective. i3wm is a window manager with no "composing" capability. This, among other things, sometimes produces a loss of performance in some graphical tasks, such as problems with smooth scrolling on the browser, and/or flashing colors before showing the rendering of the desktop. (In your case is white.)
So you could obtain a compositor for i3wm. Compton is an option that seems popular among the community. I haven't tried myself, but the lack of compositor may be the root of your problem. Here there is (an interesting related question) in i3wm FAQs. This is part of the answer:
i3 depends on an external application for compositing and Compton is an excellent choice if you want to improve rendering quality or apply hardware-accelerated translucency effects.
There are two issues I am aware of that affect stock compositing. One is screen tearing, which you may notice with animated effects such as Firefox's smooth scrolling. Another is a flash of partially rendered content when switching workspaces, or opening and closing windows. Using Compton should resolve those issues if it is configured correctly.