Thunderbird for desktop should also support tags without a color (ie transparent). You can apply multiple tags to a message, and then the message on the list is displayed in the color of the first tag alphabetically. Eg if you tag a message as 'Later' (say, yellow) and 'Travel' (say, orange), the message will appear in yellow, but you may want 'Travel' to be the dominant tag. Currently, I would rename 'Later' as 'zLater' to make sure it is listed last and deprioritised, but it gets more complicated when you want to apply several tags to a message.
A transparent 'Later' could be applied without affecting the display color, but 'Later' could still be used in filtering rules etc.
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