Generate lighter/darker color in css using javascript

Question

Hi, let say I have #404040 color code. How to generate a new color code which is either lighter or darker by 20% (or given x%)? I need this for generate a hover color in a dynamic site (which color is changing using theme). Therefore it is not possible to use another class or :hover with predefined class.

Thanks

Answer 1

var pad = function(num, totalChars) {
    var pad = '0';
    num = num + '';
    while (num.length < totalChars) {
        num = pad + num;
    }
    return num;
};

// Ratio is between 0 and 1
var changeColor = function(color, ratio, darker) {
    // Trim trailing/leading whitespace
    color = color.replace(/^\s*|\s*$/, '');

    // Expand three-digit hex
    color = color.replace(
        /^#?([a-f0-9])([a-f0-9])([a-f0-9])$/i,
        '#$1$1$2$2$3$3'
    );

    // Calculate ratio
    var difference = Math.round(ratio * 256) * (darker ? -1 : 1),
        // Determine if input is RGB(A)
        rgb = color.match(new RegExp('^rgba?\\(\\s*' +
            '(\\d|[1-9]\\d|1\\d{2}|2[0-4][0-9]|25[0-5])' +
            '\\s*,\\s*' +
            '(\\d|[1-9]\\d|1\\d{2}|2[0-4][0-9]|25[0-5])' +
            '\\s*,\\s*' +
            '(\\d|[1-9]\\d|1\\d{2}|2[0-4][0-9]|25[0-5])' +
            '(?:\\s*,\\s*' +
            '(0|1|0?\\.\\d+))?' +
            '\\s*\\)$'
        , 'i')),
        alpha = !!rgb && rgb[4] != null ? rgb[4] : null,

        // Convert hex to decimal
        decimal = !!rgb? [rgb[1], rgb[2], rgb[3]] : color.replace(
            /^#?([a-f0-9][a-f0-9])([a-f0-9][a-f0-9])([a-f0-9][a-f0-9])/i,
            function() {
                return parseInt(arguments[1], 16) + ',' +
                    parseInt(arguments[2], 16) + ',' +
                    parseInt(arguments[3], 16);
            }
        ).split(/,/),
        returnValue;

    // Return RGB(A)
    return !!rgb ?
        'rgb' + (alpha !== null ? 'a' : '') + '(' +
            Math[darker ? 'max' : 'min'](
                parseInt(decimal[0], 10) + difference, darker ? 0 : 255
            ) + ', ' +
            Math[darker ? 'max' : 'min'](
                parseInt(decimal[1], 10) + difference, darker ? 0 : 255
            ) + ', ' +
            Math[darker ? 'max' : 'min'](
                parseInt(decimal[2], 10) + difference, darker ? 0 : 255
            ) +
            (alpha !== null ? ', ' + alpha : '') +
            ')' :
        // Return hex
        [
            '#',
            pad(Math[darker ? 'max' : 'min'](
                parseInt(decimal[0], 10) + difference, darker ? 0 : 255
            ).toString(16), 2),
            pad(Math[darker ? 'max' : 'min'](
                parseInt(decimal[1], 10) + difference, darker ? 0 : 255
            ).toString(16), 2),
            pad(Math[darker ? 'max' : 'min'](
                parseInt(decimal[2], 10) + difference, darker ? 0 : 255
            ).toString(16), 2)
        ].join('');
};
var lighterColor = function(color, ratio) {
    return changeColor(color, ratio, false);
};
var darkerColor = function(color, ratio) {
    return changeColor(color, ratio, true);
};

// Use
var darker = darkerColor('rgba(80, 75, 52, .5)', .2);
var lighter = lighterColor('rgba(80, 75, 52, .5)', .2);

Now handles RGB(A) input, as well as hex (3 digit or 6).

Answer 2

You basically just need to add (for lighter) or subtract (for darker) equal amounts from each of R, G, B components.

Take a look at Domino 2.0 which is a small javascript library that does just that.

Answer 3

You could make a partially-transparent white or black PNG and overlay (underlay?) it on hover:

div.button {
  background-color: #404040;
}
body>div.button:hover {
  background-image: url('blackpixel.png');
}

No JS required.

Answer 4

This is just a modification of eyelidlessness' answer, since I saw the same function twice

var pad = function(num, totalChars) {
    var pad = '0';
    num = num + '';
    while (num.length < totalChars) {
        num = pad + num;
    }
    return num;
};

// Ratio is between 0 and 1
var changeColor = function(color, ratio, darker) {
    var difference = Math.round(ratio * 255) * (darker ? -1 : 1),
        decimal    = color.replace(
            /^#?([a-z0-9][a-z0-9])([a-z0-9][a-z0-9])([a-z0-9][a-z0-9])/i,
            function() {
                return parseInt(arguments[1], 16) + ',' +
                    parseInt(arguments[2], 16) + ',' +
                    parseInt(arguments[3], 16);
            }
        ).split(/,/);
    return [
        '#',
        pad(Math.max(parseInt(decimal[0], 10) + difference, 0).toString(16), 2),
        pad(Math.max(parseInt(decimal[1], 10) + difference, 0).toString(16), 2),
        pad(Math.max(parseInt(decimal[2], 10) + difference, 0).toString(16), 2)
    ].join('');
};
var lighterColor = function(color, ratio) {
    return changeColor(color, ratio, false);
};
var darkerColor = function(color, ratio) {
    return changeColor(color, ratio, true);
};

// Use
var darker = darkerColor('#404040', .2);
var lighter = lighterColor('#404040', .2);