Why do so many old displays/indicators particularly use red lights? Are/Were white diodes harder to create?
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In addition to what others have said, red LEDs still have one advantage: They need a lower voltage than the other colors. You need ~1.7V for a red LED but ~3V for a blue LED.
When devices were mainly powered by 2×1.5V alkaline batteries in series (or a 3V lithium coin cell) you only had around 3V when the batteries were still fresh (or even only 2.4V with two NiMH cells). Today we often use lithium-ion batteries with 3.7V.
It’s more difficult to raise voltage (you need a boost converter), so if you can you try to stay below battery voltage. With lots of battery setups this meant going for red or green LEDs.
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