Newtonian mechanics or Maxwell's equations do not explain why some substances emit only light of selected colors. When it comes to the description of small objects such as the atoms, we need a more evolved formalism called Quantum Mechanics. In quantum mechanics, all the information concerning the state of a system is contained in an object called the wavefunction. If two wavefunctions are added, one obtains a new wavefunction. This is called the superposition principle. In quantum mechanics, it is impossible to construct a wavefunction where position and velocity are precisely defined at the same time. This intrinsic limitation is called uncertainty principle. These concepts were developed in articles published between 1925 and 1930 by physicists like W. Heisenberg, L. de Broglie, P. Dirac, N. Bohr and E. Schrödinger. Reference: Chapter 15 of Chance and Chaos. A nice introduction to the basic principles and results of quantum mechanics can be found in Feynman's lecture volume 1, chapters 37 and 38.