Human eyes have three kinds of cells (photoreceptors) for color detection. They each react to either red, green or blue light. If more than one of those cells are activated, your brain interprets the light based on what cells activated, and how strongly they activated. If red and green cells activates, the light is seen as yellow. The light is seen as white if all of them activates fully.

This also means that light bulbs can produce white light by simply producing three wavelengths (colors) of light. The problem with that kind of “fake” white is that colors will look wrong under such light due to the way how objects reflects light. This is very common with low quality LED lights, and even the best smart lights aren't very good at it. When buying LED lights, you might want to look at the CRI (Color Rendering Index) value and make sure it's above 90, or as high as possible.