So how do you apply the EQ anyways?
Yeah, I'm making this last part up as I go. essentially adding colour to a track, is to change it's sonic characteristics in a very broad, Undefined way. Imagine adding a layer of saturation to your overall mix to help blend it together. Imagine adding some reverb to your guitar and vocals to put them in the same 'space.' Using some amp-simulation, re-amping, using guitar plugins. are you duplicating that guitar or vocal track and adding some sonic characteristics?, If you're doing this you're changing the 'colour of your sound.'
There are an unlimited number of things you can do with the concept of colour. It depends what you're trying to do with your song.