I don’t know what I did, but that red exclamation mark button used to add colours to your palette has disappeared! How can I get it back? Thank you!!!
The red exclamation mark only appears if your currently selected colour is not in the palette. Perhaps your current colour happens to be in the palette already?
Nope, no matter what colour I choose, it doesn’t appear. 
Hi there @MetroBunny, it’s probable that in your case you have the “Edit Color” option enabled:
So when you choose a new color it’s being changed in the palette. Pressing the A key or pressing that little button in the color bar should disable the Edit Color mode and the exclamation mark button to add new colors should appear again:
