That is in fact yellow. What OP is trying to say is that on the screen you are looking at, rather than having a LED that represents yellow, it's actually the combination of red diodes and green diodes firing at different intensities. This is how light works, as opposed to how paint works, as the two are different. YouTube has some great videos of lasers and/or prisms showing this in action.

This is simmilar to how our eyes work, where we detect varying degrees of wavelengths of light. We have 3 types of cones, one for much of blue, one for green, and for one that is much of red. The brain interprets the combination of these wavelength as the rest of the colour spectrum that you interpret. If you notice, I said much of for red and green, we see only part of the light spectrum, which is why you have heard of the terms ultraviolet and infrared. There are wavelengths we can detect, but cannot see physically because we lack the cones to detect them (though we are able to detect them with technology).

Regardless of what some of the comments stipulate, web colours are all represented by RGB values. There is another standard with print, CMYK, which is a combination of 3 colours (cyan, magenta, yellow) and black. This aligns with what you're familiar with concerning paint (kinda). It's impossible to represent black on a digital display, as black is the absence of color, and white being all colours combined.

On cellphones with OLED, rather than representing blacks as a low saturation of RGB, the led turns off entirely, and this is what created deeper blacks. Where with a cheaper LCD, the backlight is still active behind the low intensity color blend, so you see a grey as opposed to black.