Having changed my desktop image I now find the main panel (where sytray etc is) has transparency set on and I want to turn it off and set a fixed background solid colour for it. However I am unable yet to find how to do this or if it is even possible. Does anyone know if it can be done easily and if so how?
Stuart
I think you get solid colour if you turn off compositor in systemsettings>desktop and monitor. I’m not sure if you can choose the colour. I suspect it would just be part of the sytem colours you are using. I haven’t tried so I apologise in advance if I’ve got it wrong.
Panel transparency and colour are governed by the desktop theme, currently there’s no way to change that, (easily, as an end user).
As @peteh100 wrote, turning of compositing will remove transparency, but it will also effectively disable other desktop effects that you may want.
You could take a look around the KDE “store” ( https://store.kde.org/ ) to see if there’s a desktop theme more suited to your needs.
Thanks for the updates. Unless I needed to reboot or logoff turning off the compositor made no difference. Anyway I can live with what I have as it is only a very minor annoyance.
Stuart
If you changed it from “System Settings -> Display and Monitor -> Compositor -> Enable compositor on startup” then a log out / log in is required.
Alternatively you can toggle the state using Alt-Shift-F12, that takes effect immediately, but will only be for the duration of the KDE session.
Actually, it’s not hard. Previously (KDE4?) you could combine individual items from your installed themes to compose your own. This option have been removed from system-settings, but you still can do it by just copying other theme items you want into your current theme folder.
For example, I like all the Oxyzone theme, except the panel background that is too transparent, much like the OP complaint.
But I do like the panel from the Elegance theme, so it’s just a matter setting Oxyzone as your theme and copying elegance’s
panel-background.svg
from
~/.local/share/plasma/desktoptheme/Elegance
to
~/.local/share/plasma/desktoptheme/Oxyzone
Restart the desktop session, et voilà. 
Yes, I quite missed the old (KDE4) “mix ‘n’ match” of theme elements. If I recall correctly it wasn’t ported to plasma 5 because the developers claimed the module that handled that was rather buggy… Always worked for me though ;)…
If you want to start modifying themes you’ll probably find this useful:
https://techbase.kde.org/Development/Tutorials/Plasma5/ThemeDetails