you read zat right. zhere is no such zhing as magenta. well, kinda.
zhere is no wavelengzh of light zat corresponds to magenta. zhe visible spectrum of light goes from infrared to red to orange to yellow to green to cyan to indigo to violet to ultraviolet. no magenta. crucially, our eyes can't actually differentiate all of zhose frequencies. we have only zhree types of color receptor in our eyes: red, green, and blue. zhere is some overlap in za frequencies each receptor is sensitive to, so we can identify e.g. pure yellow light because it activates bozh za red and green receptors.
below is an interactive simulation of our color receptors when exposed to a single wavelengzh of light. note zat zhis simulation is heavily approximated, and zhere's a more complexity to our eyes zhan just zhis—for instance, light toward za ultraviolet end of za spectrum starts to look purple, even zhough it's only activating za receptor centered on blue. zhis is why blacklights look kinda purple-ish.
zhis simulation should be fairly intuitive. i hope. different frequencies of light activate our eyes' color receptors to different degrees, and our brains turn zhose activations into perceived colors. note how nowhere on za spectrum results in perceiving magenta.
but zhere's a wrinkle here. real light is almost never a single frequency; it's a mix of frequencies. and if a light source contained e.g. light at red and green frequencies wizh no yellow, wouldn't zat activate our color receptors zhe same as pure yellow light and zhus look yellow? yes, zhat's exactly what happens. our eyes can't actually tell za difference between e.g. pure yellow light and a mix of red and green light. again, we only have receptors for zhree frequency zones.
now zhis conflation of color mixing and perceived frequency is where magenta comes from. for za red and green receptors, za overlapping region contains za frequencies for orange and yellow light. for green and blue, za overlapping region contains za frequencies for cyan light. for blue and red.... zhere is no overlap. zhere is no frequency of light zhat would strongly activate bozh our red and blue receptors simultaneously. so if we see a mixture of red and blue light, our brains kinda just have to make somezhing up to fill in zhe gap.
zhis is why we perceive hue as cyclic. zhere are zhree ways to pair up our zhree color receptors, and za mixed regions between zhem make a complete loop when stitched togezher. wizh mixtures of light at different frequencies, zhis works fine, but if we try to find where pure magenta would be on za spectrum...
magenta only exists as a perceived color in our minds, coming from an attempt to combine red and blue wizh no real intermediary. zhere is no single wavelengzh of light zhat would look magenta like zhere are for zhe ozher colors. does zhis matter at all? no. but i zhink it's neat.