Created attachment 137185 [details]
screenshot of selected area including the onion skin preview

SUMMARY

When selecting green pixels with the selection tool for selecting by color, the selection includes the preview from the onionskin. 

STEPS TO REPRODUCE
1. enter animation window
2. make two frames
3. turn on the onion skin
4. do not change the default green color in the onion skin settings window 
5. draw green on frame one
6. draw green somewhere else on frame two
7. select your green pixels with the selection tool for selecting by color on the 2nd frame. The onion skin preview from the first frame is displayed in a faded green, it will also get selected improperly.

OBSERVED RESULT

selection by color in the animation window selects onion skin area if the colors are close enough. This effected me with the green default color.

EXPECTED RESULT

I would not expect the onion skin area to be included, since it contains area that you probably would not want selected for applying new color on the particular layer you are editing. There could possibly be a use for selecteding the general outline of all the frames stacked on each other, but I can't come up with any ideas on how to use it. It looks like a bug.

SOFTWARE/OS VERSIONS
Windows: Windows 10
macOS: 
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 
Qt Version: 

ADDITIONAL INFORMATION