Click on the little 'face' in the toolbar to open the company info window.
In the company info window, click "Colour Scheme".
Try changing the Standard Livery from there. Does it update? (It does for me)

The setting you were most likely trying to change in game is "Starting company color", but that only affects new games and when you start a new (human) company in an existing game.

Hi, I have tested all sorts of things from single player to LAN game. Once the game starts it is not possible to select any other colour scheme. It's like they do not exist in the save game or something.

Even starting a brand new game, once the game has started I can no longer change scheme.

As James says, it's not clear what you've tried and what isn't working. See the tutorial for how you change your company colour in-game:

https://wiki.openttd.org/en/Manual/Transport%20company + https://wiki.openttd.org/en/Archive/Manual/Company%20Colours

I have done the following:

1. Installed OpenTTD from official.deb version 12.1 for Ubuntu 20.04
2. I start a new single player game with no NewGFX loaded.
3. Once the game starts it is not possible to change colour scheme.

Although, I tried lots of other settings, to determine if I had done something wrong, with these 3 basic steps above, it is possible to reproduce the problem.

In the past, I used different colour schemes to label and rank different groups of vehicles. For example, all my passenger vehicles were white scheme, while my Oil vehicles were Purple.

It is still not clear what you are doing. Which buttons are you pressing, what is not happening? Are you able to select different colour schemes but the vehicles aren't actually changing? Are you not able to select anything at all?

Vehicle groups can have their liveries changed via the group gui ( https://wiki.openttd.org/en/Manual/Vehicle%20groups#using-groups ) but I believe you need a NewGRF that supports that loaded before it is available (this is unchanged)

When I open the Company Window and select Colour scheme, the only scheme listed is the one selected at the start of the game, or random. But no other schemes are listed, there is no drop down list available.

It does not look like this? How does it look then?

I cannot select the drop down list because it does not appear.

When I click where it says "Green" nothing happens.

Which of them? This one is the right one:

Oh .. Well, now don't I feel sheepish!

Sorry, yes, now the drop down list appears.

No keep this open, bad UX is also a bug.

I was drawn to the bright colours.

Summary:

• heading selectable-low-light
• setting unselectable-HIGH-light

Is in-human. I was beyond confused, I was confounded ! 😄

There are many words that could be applied to company colour UI.

'Exists' might be my favourite, and the kindest.

We have no mockups or similar for anything better though, and certain behaviours are now ingrained and expected, so https://xkcd.com/1172/ will apply as usual.

Resolved by #11617. 🙂