Quantum electrodynamics is the best description of the observed behavior of electrons, light, similar and related phenomena. It reduces in appropriate limits to the 1865 Maxwell equations which describe in precise detail most classic optical observations.
In vacuum, light is freely propagating massless phenomenon which carries momentum and energy. Thus it obeys:
$$ m = 0 \\ E^2 = (mc^2)^2 + (pc)^2 = p^2 c^2 \\ E v = c^2 p \quad \Rightarrow \quad \frac{\partial E}{\partial p} = v_{\textrm{group}} = \frac{c^2 p}{E} = c = v_{\textrm{phase}} = \frac{E}{p} $$
Thus, in vacuum, there is no dispersion of a light signal with multiple components because both $$v_{\textrm{group}} $$ and $$v_{\textrm{phase}}$$ are the same.
Light signals propagate unchanged through the vacuum, subject only to the limits of collamination -- even a very good laser pointed at the moon spreads out into a spot measured in kilometers so it takes a strong signal to be used over interplanetary distances.
Visible light is that light which by non-destructive linear photochemistry potentially gives rise to the sensation of image for humans. For humans, this means light with a frequency of between about 400 THz to about 789 THz and wavelength in vacuum about 380 nm to about 750 nm (and roughly the same in air).
When visible light is restricted by a narrow band-pass filter, we can see that the eye has a specific color sensation associated with illumination in every spectral band from about 400 THz to about 789 THz and these sensations we call the spectral colors. The eye's response to a single with multiple spectral components, testing reveals, is remarkably linear. Thus (to high precision ) all sensations of color are found on the convex hull (in some space) of the sensations produced by pure spectral colors at various intensities. As it turns out, this space has about 3 principle components -- so scientific measurement of color at this level of precision happens in an abstract three dimensional space. In this space red + green = yellow because that is the outcome of mixing equal amounts of pure spectral red and pure spectral green. But the yellow has a property distinct from color, it's intensity. Since 1 + 1 = 2, the yellow formed this way is more intense than the yellow formed by 1 measure of pure spectral yellow. So color and intensity are two dimensions in this three-dimensional space. Color may be broken up in various ways -- Yellow-verus-Blue and Red-versus-GreenishBlue (called UV in the YUV color space), or Cyan-versus-Orange and Green-versus-Purple (called IQ in the YIQ color space, part of the NTSC standard and why Apple II (1970's computer) hires display had such weird color choices available), or in analogy with polar coordinates, in terms of Hue (spectral colors and some colors between red and violet which are not pure spectral colors) and Saturation (percentage of the way from a central white point to pure hue).
Computer representation of colors may be in terms of an abstract color model on in terms of reproduction using standard Red, Green and Blue phosphors or printing inks. Software built using knowledge of the human perceptual color model as revealed by experiment allows close reproduction of colors specified with some convention to be reproduced on various devices, and also allows precise paint color matching at hardware and paint stores.
Human perception of the color of objects is based on autonomous estimation of illumination and the actual signal returned to the eye. Since fluorescent and incandescent illumination differs spectrally, while having similar whitish color, fabrics may take on unfamiliar hues in unfamiliar illumination. The most extreme common example is low pressure sodium bulbs which reduce the world to a monochrome in shades of yellow. This autonomous estimation of illumination may be fooled with a border about the object one is trying to estimate in which case the same gray square may seem perceptually different against a white or black background. Recently there was a black on light blue dress photographed with a blue background which caused many people to insist (over the claims asserted by the manufacturer) that the dress was gold on white.
When objects absorb light of one frequency and then quickly emit light of a lower frequency, we call this fluorescence. Florescent colors are brighter at certain spectral bands than a pure white object would be. This is part of the reason why day-glo is another name fore these colors. Florescent bulbs are actually mercury bulbs which use the coating on the tubes to change some of the light to a whiter hue. When objects absorb light of one frequency and then slowly emit light of a lower frequency, we call this phosphorescence.
In addition to visible light, there are many forms of invisible light. When we wish to be formal we refer to all of these phenomena collectively as electromagnetic radiation as per Maxwell's laws of electromagnetism, but here I simply use light. Our description of invisible light is also broken up by frequency band because the momentum and energy of individual photons are proportional to frequency (which has been known for over 100 years), and just as the reversible photochemical action of our eyes is frequency-dependent many elements of chemistry and physics are energy-dependent. So gamma rays can perturb nuclei, x-rays and ultraviolet (a different UV) can cause irreversible photochemical changes, infrared is associated with molecular freedom to vibrate, microwaves with molecular freedoms to rotate, etc.
Spanning all of electromagnetism is the thermodynamic emission spectrum. Hot things glow. Black-colored hot things glow brighter than white-colored things. (not a typo!) Hotter things glow brighter than cooler things. The hue of successively hotter things starts red and grows oranger, then yellow then whitish then blue-whitish. The mathematical formulas for the spectrum and intensity of hot things has been known in detail for over a hundred years the mathematical ideal is called the blackbody spectrum. For objects which are not perfectly black, they achieve this only in approximation proportional to how black they are at every spectral band -- this is called their greybody spectrum.
