In today's screens, each individual pixel is created by the combination of three transistor as Red Green Blue. But, colours are already just different frequency waves. Why aren't pixels made by just one single component to create different colour values? Is it cost or capability of electronic components?
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For each pixel, manufacturers are putting three different colour generating transistors/materials, and by the combination of those colours, we can have millions of different colours. My thought was that if colour is a wave as a photon, and colour of photon changes according to the frequency of it, we could use just one transistor/material for each pixel and change its frequency to have any colour we want. But as explained in comments, colours are very high frequency waves (3-digit Terahertz values), and technologically we are not able to generate those frequencies yet, which explains why it is not done in that way.