Eli5: What is the purpose of the different colored bars on the tv error screen? Why not just have a black screen that says ‘error’?
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The general concept is called a [Test card](https://en.wikipedia.org/wiki/Test_card), which originated as a pattern broadcast during the night hours in the early eras of TV before 24-7 programming. It was a known pattern that could be used to adjust a cathode-ray tube TV’s calibration. When TV stations went to 24-hour programming, test patterns stuck around as an established convention for “Our signal is still broadcasting but our content is off the air”—just used during technical difficulties instead of nightly.
[The specific pattern of colored bars](https://en.wikipedia.org/wiki/SMPTE_color_bars) was developed for testing the accuracy of color information being broadcast. Engineers know what the test pattern *should* look like, and by measuring the colors on the receiving end, they can figure out what adjustments to make on the broadcast end to ensure a good picture for viewers.
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