In the last few weeks, I have had a few occasions when the colouration of the Windows 10 taskbar and its Star Menu has departed from my expectations. At times, this happened in VirtualBox virtual machine installations and both the legacy 5.2.x versions and the current 6.x ones have thrown up issues.
The first one actually happened with a Windows 10 installation in VirtualBox 5.2.x when the taskbar changed colour to light grey and there was no way to get it to pick up the colour of the desktop image to become blue instead. The solution was to change the Windows from Light to Dark in order for the desired colouration to be applied and the settings above are taken from the screen that appears on going to Settings > Personalisation > Colours.
The second issue appeared in Windows 10 Professional installation in VirtualBox 6.0.x when the taskbar and Start Menu turned transparent after an updated. This virtual machine is used to see what is coming in the slow ring of Windows Insider so some rough edges could be expected. The solution here was to turn off 3D acceleration in the Display pane of the VM settings after shutting it down. Starting it again showed that all was back as expected.
Both resolutions took a share of time to find and there was a deal of experimentation needed too. Once identified, they addressed the issues as desired so I am recording here for use by others as much as future reference for myself.