Gmail – Archiving old googlemail labels

The workaround that I suggested in the question,

"clear the color of an existing custom color label (freeing up a color slot) and then change the color of the label to one of the default colors (taking up a color slot) and crucially making that default color available for other labels"

Can be achieved through the Gmail user interface by following the steps below (make sure that you perform step 3; failing to perform step 3 initially made me think that this workaround did not work).

1. Find a label with a custom color.
2. In the label-options pop-up, click "Remove color"
3. REFRESH YOUR BROWSER - the color slot that you just made available can only be used after a browser refresh.
4. Find a label with no color.
5. In the label-options pop-up, set the label color to a default color.

A message will now appear,

"The color for the label was adjusted".

The label will become colored and this default color is now available for other labels. Problem solved! :-)

Repeat this process for every default color that you want to restore.

In order to restore all 24 default colors quickly, do the following,

1. Clear 24 custom colors.
2. Remember to refresh your browser after removing the custom colors.
3. Then create 24 new labels (named "label1", "label2", "label3" et cetera - you can give the labels more obscure names if you don't want them to appear in suggested search).
4. Set each label to a different default color.

As long as you keep these labels, all 24 default colors will be available for you to use with other labels. If Gmail ever changes their default color values again, then you can update the colors on these 24 labels to match.

You can use Gmail's Muting feature to automatically have subsequent messages bypass the inbox, but that won't automatically apply the label. For that, you would need to create a filter action based on the subject line.