I have a "white back iluminated" white background footage of a person talking. I use a sort of "big softbox" as a iluminated white background behind the person talking.
In the person's clip, I draw a garbage matte because around the person not all the background is "100% white", just the "persons area" in frame.
Them I create a "New item / Color matte" and set it to pure white "FFFFFF", and place it in a lower video track, below the video track with the clip of the person with the garbage matte.
So, the "100% white" zone around the original footage of the person + the "100% white" color matte item = the person appears in an "infinite perfect white background".
So far, so good, the background looks "infinite perfect" and shows no banding or any color issues or anything in Premiere.
Them I export in .mp4 H264 (no mater if I use or not any of the "Youtube export presets" in the Media Encoder, makes no difference), if I play the .mp4, it looks OK....
...But and when I upload to Youtube it shows a strange "purple halo" and the strange and certainly unexpected strong difference 😲 from the "white" between the "Pure white FFFFFF" Color Matte element (which looks almost "gray" in Youtube), and the "real white" around of the person in the original footage, with the garbage matte.
The footage of the person is graded with Lumetri, but nothing to fancy, just a little color correction and that's all.
In this screen capture of how the video looks in Youtube, I point the "purple halo" around the garbage matte, and also a strange "artifact" that also appears near the neck of the person :
What am I doing wrong making the "garbage matte" thing to generate a perfect white background 😓 ?
Remember that that "gray background" zones are in fact the "100% white FFFFFF" "Color matte" element created in Premiere. But I don't know why it looks gray when I upload the .mp4 to Youtube !
For starters, I wouldn't make your color matte 100% pure white because the white seamless you have behind the person on camera probably didn't record as perfect white. So I would say your first step is using the "eyedropper" when selecting the color for your matte and selecting the color of the white background behind the person on camera. This will "match" your matte to the real world color of your white background.
Additionally I would also feather the mask you're putting around your person to make sure the blend doesn't have a "seam"
I would then nest the above stacked footage and color correct the nest so that whatever changes you make for the footage of the person is also reflected in the color matter.
Another thing to consider is that white surfaces are reflective. So if your person is moving in frame, the color of the white background in camera is going to change due to the changing color of the light getting reflected from it depending on how close the person is standing to the white background.
And finally, bit rate and codec may cause a shift in color blending since it's trying to shrink the size of the video and may be eliminating some color channels to do that.