In bug 1339610 comment 2 it was pointed out that the contextualIdentity color from the API is something like "blue". But that isn't the exact colour used in the visual identity. You can see an example of this here: https://github.com/mdn/webextensions-examples/pull/179
Has there been any movement on theme colours at all? I'm hesitant to hardcode these colours due to visibility issues with themes. Our test pilot also changes the shade of blue slightly etc. There is also a slight difference between text colour and icon colour for visibility purposes however I suspect we can ignore that. The other issue with these colours is they might clash with extensions colours themselves, I'm not really sure how to suggest mitigating that. Is there an ability to expose css custom properties to web extensions? Would that get messy if they wanted to do something in a content script? At least with colour names... the extension author can actually fix these issues, granted it is still horrible.