MsKittyTail
I think different atoms have different colors. Element are substances made up of one type of atom, and they have differing color. Iron is reddish, Silver is grey, Gold is yellow, Cobalt is blue... etc.
I guess some are colorless.. Oxygen, Hydrogen...
So both!
biggrin Atoms themselves don't have color; the colors actually correspond to the energy levels that the electrons orbiting the atoms can have. Atoms bonded to other atoms end up being colors that don't seem to have any relation to the individual atoms, because the colors are tied to the energy levels which depend on the individual atoms but also depend on how the atoms are related chemically. For example, copper metal is, well, coppery in color, but copper oxide is green. Iron metal is silvery, and as you mentioned iron oxide is red.
Electrons have different energy levels they can be at, and when an electron jumps from a higher energy level to a lower one it lets off a photon that carries away the difference in energy. Since the electrons can only be at certain energy levels, there are only certain energies that the photons end up with. For photons, their energy determines what color they appear, and thus atoms appear certain colors because the light coming from them only comes in certain colors. The exact values of the energy levels, and thus the energies of the photons, changes when atoms bond to other atoms to form molecules, which is why rusts are different colors from the metals they came from.
Gases tend to be clear not because the atoms are clear but because the gas is too diffuse for us to see it. Water vapor is clear, and water is clear, but ice and snow definitely have some color to them. When you can "see" a gas, usually you are actually seeing small liquid drops, often water vapor.
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