I tried putting the PNG from the top of the page through the icon generator on this page:
https://realfavicongenerator.net/
When it's shrunk down to icon size, the multicoloured CP doesn't look great IMO.
I played around with it and made these two versions and ran them through the icon generator, and I think they look better at the smaller size...
I didn't generate the icon files because you need access to the site settings on the server to implement them, but if you plug those images into the site linked above, it can generate all the icons that are needed to work with all the different devices and browsers that are out there.