You could just have Googled this question.
Post #5 from the first hit:
First off, by "wondering what colors different gas giants can be", you
are presumably asking about their light spectra through the visible
range of wavelengths (380-720 nm), right?**
Light interacts primarily with electrons. It is scattered or absorbed
in the presence of electrons, which come in a variety of "phases".
Here are the most relevant:
- free within an ionized gas (can absorb in the presence of the
electric field of an ion)
- attached to atoms and ions
- attached to molecules
- attached to molecules that have condensed into solid state aerosols and grains, or liquid droplets.
The most important thing to take away from this is that every type of
material, in terms of composition and "phase", absorbs and scatters
The prevalence and importance of each of the above four "phases"
depend on (a) the elemental composition of the giant planet's
atmosphere (defined as that layer responsible for light
reflected/emitted by planet), and (b) its equation of state (how
pressure changes as a function of density and temperature). The first
of these provides the raw materials, and the second arranges them in
"phase". Very roughly speaking, one may assign decreasing temperatures
(T) to the above 4 "phases" moving down the list 1-->4. Pressure (P)
also plays a role, and in general one may place the above phases on a
P-T diagram. Physics and Chemistry are at work to determine what kind
of "stuff" is present as a function of depth through the giant planet
atmosphere. One rule of thumb is that chemistry is much more effective
at higher temperatures (to a point) and/or in the presence of
moderately energetic light.
Next, before proceeding, go back and read the bold statement, above.
To finish off this overly long post: Two giant planets of equal bulk
compositions will almost certainly appear differently if their P vs. T
profiles differ, or if their atmospheric compositions differ (e.g.,
due to convective mixing from the interior, mixing due to wind
currents, heterogeneous settling of heavier matter towards the center
over time). Two giant planets of equal bulk compositions, but
differing ages will appear differently, since a planet's interior
cools over time, affecting P-T relation within the planet as well as
its thermally emitted spectrum. The intensity and spectral shape of
the light incident from the parent star will affect the P-T diagram,
the chemistry and phase of the matter, the thermally emitted spectrum,
as well as the distribution of photons available for scattering.
From the second hit:
Jupiter is a giant gas planet with an outer atmosphere that is mostly
hydrogen and helium with small amounts of water droplets, ice
crystals, ammonia crystals, and other elements. Clouds of these
elements create shades of white, orange, brown and red. Saturn is also
a giant gas planet with an outer atmosphere that is mostly hydrogen
and helium. Its atmosphere has traces of ammonia, phosphine, water
vapor, and hydrocarbons giving it a yellowish-brown color. Uranus is a
gas planet which has a lot of methane gas mixed in with its mainly
hydrogen and helium atmosphere. This methane gas gives Uranus a
greenish blue color Neptune also has some methane gas in its mainly
hydrogen and helium atmosphere, giving it a bluish color