Sound colour
What do people mean when they say X is good, but it ‘‘colours the sound’’? 
eg. when talking about a particular brand of headphone.
Thanks. 
It means it changes certain frequencies of the sound in a certain way. For example, vinyl can sound different to digital, different mixers can accentuate certain frequency bands or whatever.
Take for example traktor’s different eq modes, which emulate different mixer types.
Some systems sound warmer, some are a little harsh on the highs, sound colour is dependent on every part of your setup.
Think of it as “changes the sound”.
It’s basically similar to what you do when you’re DJing using the EQ knobs. You’re essentially “colouring” the sound by adjusting the EQ, i.e. frequencies.
yeah, a change of frequencies. I think the phrase derived from the fact that different colours of light have different frequencies.
Colouration is can be spoken of in a positive or negative light. It means there is a deviation from a flat response or a deviation from no distortion to distortion. i.e. A nice analogue compressor with tubes and transformers in can “colour” the sound in a pleasing retro way.
cheers