gistfile1.js

function getRGB(color) {
    // http://plugins.jquery.com/files/jquery.color.js.txt
    var result;

    // Check if we're already dealing with an array of colors
    if (color && color.constructor == Array && color.length == 3)
        return color;

    // Look for rgb(num,num,num)
    if (result = /rgb\(\s*([0-9]{1,3})\s*,\s*([0-9]{1,3})\s*,\s*([0-9]{1,3})\s*\)/.exec(color))
        return [parseInt(result[1]), parseInt(result[2]), parseInt(result[3])];

    // Look for rgb(num%,num%,num%)
    if (result = /rgb\(\s*([0-9]+(?:\.[0-9]+)?)\%\s*,\s*([0-9]+(?:\.[0-9]+)?)\%\s*,\s*([0-9]+(?:\.[0-9]+)?)\%\s*\)/.exec(color))
        return [parseFloat(result[1])*2.55, parseFloat(result[2])*2.55, parseFloat(result[3])*2.55];

    // Look for #a0b1c2
    if (result = /#([a-fA-F0-9]{2})([a-fA-F0-9]{2})([a-fA-F0-9]{2})/.exec(color))
        return [parseInt(result[1],16), parseInt(result[2],16), parseInt(result[3],16)];

    // Look for #fff
    if (result = /#([a-fA-F0-9])([a-fA-F0-9])([a-fA-F0-9])/.exec(color))
        return [parseInt(result[1]+result[1],16), parseInt(result[2]+result[2],16), parseInt(result[3]+result[3],16)];

    // Otherwise, we're most likely dealing with a named color
    return colors[jQuery.trim(color).toLowerCase()];
}
function getBrightness(color) {
    // http://stackoverflow.com/questions/596216/formula-to-determine-brightness-of-rgb-color
    var rgb = getRGB(color);
    var r = rgb[0]/255;
    var g = rgb[1]/255;
    var b = rgb[2]/255;
    return Math.sqrt(0.241*r*r + 0.691*g*g + 0.068*b*b);
}

$('.colours input').each(function() {
    // it would probably be better to set "bright" and "dim" classes rather than hardcoding the colours here
    var $this = $(this);
    var bg = $this.val();
    var brightness = getBrightness(bg);
    // I find that 0.75 works a bit better as a cut-off than 0.5
    var fg = (brightness > 0.75) ? '#000000' : '#ffffff';
    $this.css('background-color', bg);
    $this.css('color', fg);
    var bordercolor = (brightness > 0.75) ? '#cccccc' : bg;
    $this.css('border-color', bordercolor);
});