Concrete is a shade of grey. Grey, as a leading colour, lends a formal and serious impression to a given building. It better serves an impactful supporting colour (but only with cooler colour tones) since concrete does not stand out on its own. Concrete is perfect to mix with cool reds/raspberry, or other muted versions of these shades. Or works well with cool green-yellow tones such as grapefruit, as well as other lighter versions of grey.