What does the color of the “L” box in the top left mean?
[This] has been discussed before!
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That’s the logo, much the way Hacker News has Y for YCombinator. Lobsters has L.
I think the OP means, why is it red sometimes and black others.
AFAIK it is traffic: https://lobste.rs/s/xgn8rn/does_the_lobsters_l_logo_change_colors.
It is an indicator of traffic. See application.html.erb for details.