Thanks for getting back so quickly!
I've been playing around with icons recently, and the AppIndicator icons are just part of it. You can see some of my other mockups in this thread:
Regarding the tint, I went in with the eyedropper tool in your screenshots, and the other KDE tray icons are 94%-luminance gray. So they're not 100% white, either, but they're a slightly different tint. Electron not supporting SVG indicator icons seems to be an upstream problem with Chromium, so I was thinking of creating an issue on the chromium.org issue tracker. (EDIT: see here.) I would not be surprised if a proper SVG indicator icon ended up using the correct fill color on both GNOME Ubuntu and KDE.
On my open pull requests for the application icons, @laurent mentioned the lack of engagement here on the forum as a reason to hold off on approving them, so soliciting feedback from other users like you is an important part of the process (and indeed you can update GitHub pull requests with additional commits).
My thought for once I do a pull request for the tray icons in the main repository is that I will include the SVG for all three shapes but only use the plain square one in the build for the time being. The book icon I did in part because I made some book icons in the other thread, as well.