Opposite Color Tool
Two colors are considered complimentary if they produce a neutral color — black, white, or grey — when mixed evenly. But, that is a mouthful. Simply put, a complimentary color has the opposite hue of a given color.
This tool will display the opposite color from a base color, and gives the hexadecimal code for your css/html:
Alternatively, you could use the “invert” option in photoshop to get the same results…