when we say that a lemon is yellow what we actually mean is that the lemon absorbs all the color except yellow and it reflects yellow which fall in our retina which we perceives as yellow.
but the screen that we use does not produce yellow at all. in fact it can only produce red, green, and blue light. but then how do we see yellow. the answer is that, it is really simple to lie to our brain. inside our retina we have 3 types of cone cell (which are sensitive to light). each one is best suited to detect a certain color. one is great for blue the other is great for green and the last one is great for red so the way we see yellow is that the wavelength of yellow light falls between the wave length of green and the wavelength of red. so when a yellow light come to our retina it slightly activates the red and the green cones which our brain says that what happens when something is yellow so it must be yellow
all what a monitor has to do to make you think that you are seeing yellow is to send a little bit of green and red. when the pixels are small enough that they cannot be individually be distinguished the brain thinks that i am receiving some red and some green thats what yellow things do therefore it must be yellow even though it actually is not
this is what we see as yellow