Imagine that we have a special recipe to synthesize Quantum Dots by mixing only two types of atoms, which always results in spherical particles. At the start of this synthesis, atoms begin to aggregate to form the first seeds of Quantum Dots. When left to cook for longer, more atoms are added to these Quantum Dot seeds, allowing them to grow larger. The small Quantum Dots at the start of this synthesis and the larger Quantum Dots obtained at the end are of the same composition, same form, but of different sizes. This difference in size will result in a different emitted wavelength of light, thus a different color. Hence, if our Quantum Dots emit within the visible spectrum of light, smaller Quantum Dots will provide a shorter wavelength, or color closer to blue, while larger Quantum Dots will provide a longer wavelength, or color closer to red.
However, this process is not limited to only the visible spectrum. It is possible to create much larger Quantum Dots that emit in the infrared, or much smaller Quantum Dots that emit in the ultraviolet range.