Why are some tags grey and some are light grey?

Question

I'm here to ask why are some tags grey and some are light grey? See in this photo:

is light grey, but is dark grey. Why is that?

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Edit: it does it here with the tags of this question

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Answer 1

Meta sites have compulsory tags, of at least one you need to have.

bug and discussion are compulsory. tags is not.

There's a third type of tag called a status tag that only mods and staff can use that's red.