Colouring is most common on home consumer products, like your hifi. They have "presets" which will in the simplest form alter the bass mids and treble response. If you listen to a CD on different presets, different hifis or even just different speakers (or headphones) each will "colour" the sound differently giving it a different texture. Each piece of equipment will reproduce different frequencies at different amplitudes, some purposefully (like the presets) in order to attempt to make the sound more attractive to human ears and the human hearing range.