It could be that the dtoverlay in /boot/userconfig.txt does not suit your display.
The Kuman documentation refers to the LCD35-show script from the GitHub repo of goodtft. The LCD35-show script (don’t run it ) installs goodtft’s overlay tft35a-overlay.dtb. So I would try that overlay instead of “waveshare35b-v2-overlay”. If this should not help, could you post a screenshot showing the wrong colors?
Edit: Together with @S-Informatik I have worked out a solution for a 3.5" display of Quimat using the tft35a overlay. S-Informatik has been so kind to sum up the necessary steps here. The Quimat display seems to be similar to your Kuman display so maybe that guide works for you, too.
BTW: There is no need to have the same dtoverlay entry two times in /boot/userconfig.txt (but I don’t think this causes your issue).