Light is unique among waves, in that it does not need a medium in which to propagate. Light can travel in a vacuum becuse it is a self-perpetuating wave made of oscillating electric and magnetic fields. Sound cannot travel in a vacuum - space is quiet, even if it is not portrayed that way in the movies. An Angstrom is about the size of an atom. Green and yellow light fall in the middle.
White light is a mixture of light of all colors.
The electro-magnetic spectrum includes far more than just visible light. These are the highest energy photons. Gamma rays are produced by extremely energetic events, including solar flares and terrestrial lightning, and are produced by gas at temperatures near a billion degrees. Gamma rays carry much energy, and hence are dangerous to living things because they can penetrate tissue. Wavelengths are less than about 0.01nm.