When you click on the bar at the top that says "Choose a color" - your system wide Color Chooser opens up in a separate window.

Look carefully at the Color Chooser panel in my first Screenshot. Click on the little color blob at the top (just under the Close Button - I marked it Click Me) to switch to the "Good" color pallet.

(See MY Screenshots)

Slide the slider just under the color wheel all the way to the left. If you get a custom color you want to save: drag it to an empty color chip in the bottom 2 rows. You should be able to click and drag in the color wheel to get any color you like, now.

This "solution" is a pain in the posterior. Thunderbird was working fine until a couple of days ago, then this spontaneously changed for the worse. The solution you offer means lots of extra work since the original color palette is no longer there, and as I have visual problems, I can't even see the empty boxes to put the colors in.

BTW, I had another issue with Thunderbird that also occurred spontaneously a couple of weeks ago, so I'm just wondering if Thunderbird is suffering from some sort of issue that may be resolved with an update. It seems to be making spontaneous changes to the way the program works, and this is unacceptable.

Are you using a MAC OS ? If yes, then the color selector 'Sea Mac' is discussing is the default Color Changer used on a MAC.

re: It seems to be making spontaneous changes to the way the program works,

Can you please explain what it is you are talking about? Give examples.