Onion skinning is a native Photoshop functionality but the Animator’s Toolbar Pro expands on its behaviour with two extra options bound to the Onion Skin toggle button.
Photoshop includes the Background Layer in the Onion Skin view and uses the Multiply blend mode by default. When a background colour other than white is used the overall image tends to darken while in Onion Skin Mode.
To exclude the Background Layer from the Onion Skin view you can toggle its visibility (and the document’s Transparency Grid) when the Onion Skin button is pressed.
If the Onion Skin toggle and the Background Layer’s visibility go out of sync; turn of Onion Skinning and click the “Restore visibility state” button to turn the visibility of both the Background Layer and Transparency Grid back on.