Currently, I am studying the inkjet printer in detail. I had come across the ink of printers ink. Why do they not use RGB ink? Why do they use CMYK? I have read somewhere if we mix red and green ink together it will become dark, but not yellow because of absorption. I need a scientific answer related to spectrum and frequency.
Short answer: Because sabtractive and additive color mixing are not the same.
Longer answer: With additive color mixing (most screens work like that) you get white when you mix all the colors. This means, the more colors you add, the brighter the result.
With subtractive color mixing (like printers do) you start with a white piece of paper and make it darker by applying color. Mixing all colors together results in something resembling black (often a brownish mess, that's why we often use K, a real black in print).
So if you had red, green and blue in a printer you could never get yellow, magenta or cyan. On the other hand, by putting a bit yellow into magenta, you get red...
We also have to keep in mind that the selection of the base colors used to mix the other colors together is always a compromise. We can't represent the whole visible color space in print or on a screen. Some screens and printers can cover a bigger area of the whole visible color space but none can display everything. That's why some printers use additional color inks in addition to the classic CMYK.
So it might be a good idea to learn about Color Spaces too.