# Nebula and its colors

A Nebula has bright awesome colors that include red, blue, green, orange, white, etc?

They're result of excitation of electrons.

Is there a clear explanation as to which color is attributed to an atom for Color properties in Cosmos!?

Generally speaking, green colours in a nebula are due to forbidden transitions in ionised Oxygen, though can feature the hydrogen $$\beta$$ Balmer line. Red colours can be due to Hydrogen $$\alpha$$, but there are also lines due to ionised sulphur and nitrogen that often make contributions.
If you take a spectrum of a nebula it usually completely obvious what elements are responsible, because the spectra consist of very sharp emission lines at the wavelengths that correspond to the well-known electronic transitions in particular species. An example is shown below, which is the spectrum of a typical planetary nebula. Here, the labels mean the following: OIII means oxygen with its outer two electrons removed; NII is nitrogen with its outer electron removed etc., H$$\alpha$$, $$\beta$$, $$\gamma$$ are the $$n=3 \rightarrow 2$$, $$n=4 \rightarrow 2$$ and $$n=5 \rightarrow 2$$ transition of the hydrogen Balmer series. The wavelength scale is in angstroms where 10 angstroms is 1 nm. So 5000 angstroms is the green part of the spectrum, 6500 angstroms is red and 4000 angstroms is blue.