Why do you see lemons as yellow?

The color of an object depends on the way it interacts with light. When white light hits an object, some of the light is absorbed by the object and the rest is reflected. The color of the object is determined by the wavelength of the light that is reflected.

In the case of lemons, they appear yellow because they absorb blue and red light from white light and reflect yellow light. This is why you see them as being yellow.