Find the appropriate color that works best on a given background color.

invert_color.php

<?php

/**
 * @file
 * Find the appropriate color that works best on a given background color.
 */

/**
 * Convert the standard RGB color to linear RGB.
 *
 * Returns a linear color based on the formula mentioned in
 * https://www.w3.org/TR/WCAG20/#relativeluminancedef.
 */
function convert_standard_color_to_linear($color) {
  if ($color <= 0.03928) {
    return $color / 12.92;
  }
  else {
    return pow(($color + 0.055) / 1.055, 2.4);
  }
}

/**
 * Convert a HEX value to RGB.
 *
 * @param string $hexStr
 *   The hex value to convert.
 * @param bool $returnAsString
 *   Whether to return this as a string.
 * @param string $seperator
 *   The character that will be used to separate the values.
 *
 * @return string
 *   The converted value in RGB.
 */
function hex2_rgb($hexStr, $returnAsString = FALSE, $seperator = ',') {
  // Gets a proper hex string.
  $hexStr = preg_replace("/[^0-9A-Fa-f]/", '', $hexStr);
  $rgbArray = array();
  if (strlen($hexStr) == 6) {
    // If a proper hex code, convert using bitwise operation. No overhead and is
    // faster.
    $colorVal = hexdec($hexStr);
    $rgbArray['red'] = 0xFF & ($colorVal >> 0x10);
    $rgbArray['green'] = 0xFF & ($colorVal >> 0x8);
    $rgbArray['blue'] = 0xFF & $colorVal;
  }
  elseif (strlen($hexStr) == 3) {
    // If shorthand notation, need some string manipulations.
    $rgbArray['red'] = hexdec(str_repeat(substr($hexStr, 0, 1), 2));
    $rgbArray['green'] = hexdec(str_repeat(substr($hexStr, 1, 1), 2));
    $rgbArray['blue'] = hexdec(str_repeat(substr($hexStr, 2, 1), 2));
  }
  else {
    // Invalid hex color code.
    return FALSE;
  }
  return $returnAsString ? implode($seperator, $rgbArray) : $rgbArray;
}

/**
 * Calculate the inverse color given a hex value.
 *
 * @param string $hex
 *   The hex value to invert.
 * @param bool $bw
 *   Whether to invert to only black or white.
 * @param bool $strict
 *   Set this to TRUE to find colors that match W3C guidelines.
 *
 * @return bool|string
 *   Reurns FALSE if invalid hex code, else returns the inverted color in RGB.
 */
function invert_color($hex, $bw = FALSE, $strict = FALSE) {
  // Gets a proper hex string.
  $hex = preg_replace("/[^0-9A-Fa-f]/", '', $hex);
  if ($hex[0] == '#') {
    $hex = substr($hex, 1);
  }
  // If this is a proper hex code, convert using bitwise operation. This has no
  // overhead and is faster.
  if (strlen($hex) == 6) {
    $hex = hexdec($hex);
    $r = 0xFF & ($hex >> 0x10);
    $g = 0xFF & ($hex >> 0x8);
    $b = 0xFF & $hex;
  }
  // If the value is in shorthand notation, we need to do some string
  // manipulations.
  elseif (strlen($hex) == 3) {
    $r = hexdec(str_repeat(substr($hexStr, 0, 1), 2));
    $g = hexdec(str_repeat(substr($hexStr, 1, 1), 2));
    $b = hexdec(str_repeat(substr($hexStr, 2, 1), 2));
  }
  // Invalid hex color code.
  else {
    return FALSE;
  }

  // If $strict is set, then the inverted colors will be black or white.
  if ($strict == TRUE) {
    // Based on the answer provided in this StackOverflow question
    // https://stackoverflow.com/a/3943023/112731
    $sRGB = [$r / 255, $g / 255, $b / 255];
    $lRGB = array_map("convert_standard_color_to_linear", $sRGB);
    $luminosity = 0.2126 * $lRGB[0] + 0.7152 * $lRGB[1] + 0.0722 * $lRGB[2];
    // $intensity should be > 186;
    return ($luminosity > 0.179)
        ? '#000000'
        : '#FFFFFF';
  }

  if ($bw == TRUE) {
    // Based on the answer provided in this StackOverflow question
    // https://stackoverflow.com/a/3943023/112731
    $intensity = $r * 0.299 + $g * 0.587 + $b * 0.114;

    // $intensity should be > 186;
    return ($intensity > 186)
        ? '#000000'
        : '#FFFFFF';
  }

  // Invert color components.
  $r = (string) dechex((float) 255 - $r);
  $g = (string) dechex((float) 255 - $g);
  $b = (string) dechex((float) 255 - $b);

  // Pad each value with leading zeros if required.
  $r = str_pad($r, 2, '0', STR_PAD_LEFT);
  $g = str_pad($g, 2, '0', STR_PAD_LEFT);
  $b = str_pad($b, 2, '0', STR_PAD_LEFT);
  return "#" . $r . $g . $b;
}