I'm sorry if this has been answered, I couldn't find anything about it.
When I'm editing a graphic object on the stage (not from the library) everything else is under this opaque white screen so I can't see their true color.
Is there a way to disable it?
No. It's a convenience feature to help you retain alignment and get sort of an onion skin idea of where you are.
Tou can always take a screenshot and put it in a locked background layer under your clip of your content to simulate the full color. It's all about how complex you need it to be.
If the background is animated then the good old way is the only way. Copy the frames from inside the clip (hopefully not to complex), make a new layer in the content you want to edit it in with full color, paste frames, make changes, then copy it back into your previous MovieClip.
I myself always desire that semi-transparent white to make the clips contents obvious but I have wanted to disable that before to ink dropper select some colors.