Given a background color, how to get a foreground color that makes it readable on that background color?

Here’s one I did in both Java and Javascript. It’s loosely based off this one in javascript. I took the Luminance formula from here. The sweet-spot of the threshold from my eye was about 140.

Java version:

public class Color {

    private float CalculateLuminance(ArrayList<Integer> rgb){
        return (float) (0.2126*rgb.get(0) + 0.7152*rgb.get(1) + 0.0722*rgb.get(2));
    }

    private ArrayList<Integer> HexToRBG(String colorStr) {
        ArrayList<Integer> rbg = new ArrayList<Integer>();
        rbg.add(Integer.valueOf( colorStr.substring( 1, 3 ), 16 ));
        rbg.add(Integer.valueOf( colorStr.substring( 3, 5 ), 16 ));
        rbg.add(Integer.valueOf( colorStr.substring( 5, 7 ), 16 ));
        return rbg;
    }
    public String getInverseBW(String hex_color) {
        float luminance = this.CalculateLuminance(this.HexToRBG(hex_color));
        String inverse = (luminance < 140) ? "#fff" : "#000";
        return inverse;
    }

}

Javascript version:

Here’s the same thing in javascript for your front-end things. RGB conversion taken from here:

hex_to_rgb: function(hex) {
        let result = /^#?([a-f\d]{2})([a-f\d]{2})([a-f\d]{2})$/i.exec(hex);
        return result ? { 
                r: parseInt(result[1], 16),
                g: parseInt(result[2], 16),
                b: parseInt(result[3], 16) 
        } : null;
},
hex_inverse_bw: function(hex) {
        let rgb = this.hex_to_rgb(hex);
        let luminance = (0.2126*rgb["r"] + 0.7152*rgb["g"] + 0.0722*rgb["b"]);
        return (luminance < 140) ? "#ffffff": "#000000";
}