We had an interesting experience on our project a few weeks ago. We asked the users for colour specifications for the GUI. Now bearing in mind they have been using a system that is 20 years old, they came up with "green back ground", "blacks", "grays", etc.
Here is a demo:
OK - I admit I used the brightest green I could find, but the point was that with modern look and feels, we thought it would be better to stick with default colours:
Lessons learned? Work closely with the customer but don't be afraid to recommend the sensible options when they arise.