Color does not add a pleasant quality to design — it reinforces it.
Color is one of the most powerful tools in a designer’s toolkit. It can draw attention, set a mood, and influence a users’ emotions, perceptions and actions. Did you know that color accounts for 90% of the reason why you purchased a specific product? Or that full-colored ads in magazines are recognized 26% more than black and white ads?
Nick Babich has a great article on color theory for designers—check out The impact of color on conversion rates at the Adobe Creative Cloud blog.