White light is always said to contain all the different wavelengths of light. Why, then, can we 'make' new colors simply by adding wavelengths? Is it just a matter of our perception, that, when two colors are added together, they appear to be the wavelength of another color. Or are the waves actually forming into another, differently 'waving' wave, with a different wavelength?
It is simply a matter of perception.
Human eyes have four types of receptor
So, to understand human colour perception in daylight, you only need to work with a model of light as a mixture of different frequencies which pass through one another losslessly.
If a particular source of light is strong at frequencies that mainly stimulate two types of cone, your brain interprets this as an intermediate "colour" between the peaks of each types response curve.
Note that TV screens exploit the specific mechanisms of human colour perception. They don't really need to produce an arbitrary range of frequencies, they just need to produce three specific colours at the right mix of intensities to stimulate your receptors to the same degree that an intermediate frequency would.
In my answer to this question, I explain how white light is composed of all of the wavelengths output by the Sun in the visible spectrum. I also go on to explain how we can add specific wavelengths to "simulate" white light due to the way our eyes have evolved. The specialized cone cells in our eyes are tuned to different wavelengths and by stimulating them at specific ratios, we can simulate any visible wavelength. I recommend giving it a read as it should perfectly answer your question and it has some really nice and colourful pictures.