What do the colors of the Title Bars mean?

I just filed a Wish List item here

about color-coding any node with the IsInput flag set.
In the process, I noticed something that I’ve never paid attention to before:
Some nodes have a dark gray Title Bar, and some have light gray.
Why is that, and what’s the logic behind which is light and which is dark?
I couldn’t figure out what the logic is

From the Primer.

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Thank you, sir.

I had looked at the page you linked to but hadn’t scrolled down far enough to see the States

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