Be mindful that those colours aren't the only wavelengths of light emitted by those atoms, they're just the ones that are emitted most. Atoms have a characteristic emission spectrum that include a lot of different wavelengths. [url=http://upload.wikimedia.org/wikipedia/commons/5/52/Emission_spectrum-Fe.svg]An iron ion (not sure whether this is Fe2+ or 3+) spectrum looks like this, but when you burn it you'll only see one or two colours.[/url]