English/German as always. Embedded link:

I like to hide some treats for my 12 y/o Miez. Can you spot the one she has yet missed on top of the dart wall behind the milk carton? I don’t know how I managed to throw and land it up there, and definitely would fail many times if I tried.
Miez bevorzugt es, ihre Beute auf dem Boden zu verspeisen
Ich verstecke meiner 12 jährigen Miez gerne Leckerli. Seht ihr das zwote Leckerli auf der Dartwand hinter dem Milchkarton? Keine Ahnung wie ich’s geschafft hab, dass das da landet, und es würde mich definitiv viele Versuche kosten eins absichtlich da rauf zu werfen.
Edit: useful green circle:



I’m by no means an artist, but I’ve done quite some UI/UX design, so I’ll try to give my 2 cents about colors here. You may or may not agree since i usually try to go for simpler / pragmatic approaches (not to confuse my users lol) and your painting has a lot of depth to it:
passepartout
The most obvious choice, also matching your frame, would be just plain white, but it may be a harsh contrast to the darkness of your painting (which may be preferable). Choosing an off-white / linen color may soften the contrast and match the feathers a little better. I would avoid a black one since the subject already is framed in black for the most part, and it would kind of swallow the painting or make it appear smaller than it is. So gray was chosen, maybe going for availability at the moment of framing. For this exact one though, it may give off “document container in an 80s office” vibes at first glance since it seems to have a yellowish / greenish tint to it (may also just be the camera / lighting). So maybe even a fresh (more neutral / less saturated / darker / lighter / maybe even light blueish) gray one after all.
frame
Depending on what the painting means to you, you may also look for a new frame. I think a black neutral frame would fit pretty well. Maybe even a more pompous one in a dark wood tone or golden, but that could get too much pretty fast since your painting is pretty detailed already.
Spoiler to my username below. Looking at your username, you look like a “bad dog” to me lol (one of my favourite movies tbh). It’s just an alter ego on a niche internet platform eventually though (:
passepartout
(as in french ‘passer’ = going, ‘partout’ = through everything) mostly is two things to me:
Combining these, you could get a lot of things, which is also just an expression of what I find interesting or important.