This little PHP script helps you when you need to get all the colors between two specific colors. It's usefull if you need to create a progress bar, that changes its color.

color-progress.php

#!/usr/bin/php

<?php

/**
 * This script receives two colors and the number of steps
 * of the progress and returns the different color for each
 * step.
 */

if (count($argv) < 3) {
  print "Usage: color-progress.php HEX_COLOR_1 HEX_COLOR_2 NUMBER_OF_STEPS.
Example:
color-progress.php 0088cc CCCCCC 4
";
  exit;
}

$start = $argv[1];
$end = $argv[2];
$steps = $argv[3];


$color_progress = ColorProgress::fromHex($start, $end, $steps);
print_r($color_progress->getStepColorsInHex());
//print_r($color_progress->getStepColorsInRGB());

class ColorProgress {
  private $startingColor;
  private $endingColor;
  private $steps;

  private $stepColors;
  private $increments;

  public function __construct(Color $start, Color $end, $steps) {
    $this->startingColor = $start;
    $this->endingColor = $end;
    $this->steps = $steps;
    $this->calculateIncrements();
    $this->calculateStepColors();
  }

  public static function fromHex($start, $end, $steps) {
    $startingColor = Color::fromHex($start);
    $endingColor = Color::fromHex($end);
    $instance = new self($startingColor, $endingColor, $steps);
    return $instance;
  }

  public static function fromRGB($start, $end, $steps) {
    $startingColor = Color::fromRGB($start);
    $endingColor = Color::fromRGB($end);
    $instance = new self($startingColor, $endingColor, $steps);
    return $instance;  }

  public function getStepColorsInRGB() {
    return $this->stepColors;
  }

  public function getStepColorsInHex() {
    $hexStepColors = array();
    foreach ($this->stepColors as $stepColor) {
      $color = Color::fromRGB($stepColor);
      $hexStepColors[] = $color->getHex();
    }
    return $hexStepColors;
  }

  private function calculateStepColors() {
    for ($i = 1; $i <= $this->steps; $i++) {
      $increment = array(
        $this->increments[0] * $i,
        $this->increments[1] * $i,
        $this->increments[2] * $i,
      );
      $this->stepColors[] = $this->startingColor->incrementRGB($increment);
    }
  }

  private function calculateIncrements() {
    $origin = $this->startingColor->getRGB();
    $destination = $this->endingColor->getRGB();
    $quantity = array();
    for ($i = 0; $i < 3; $i++) {
      $quantity[$i] = ($destination[$i] - $origin[$i]) / $this->steps;
    }
    $this->increments = $quantity;
  }
}

class Color {
  private $red;
  private $green;
  private $blue;

  public function __construct($red, $green, $blue) {
    $this->red = $red;
    $this->green = $green;
    $this->blue = $blue;
  }

  public static function fromRGB($color) {
    $instance = new self($color[0], $color[1], $color[2]);
    return $instance;
  }

  public static function fromHex($color) {
    $rgb = str_split($color, 2);    $instance = new self(hexdec($rgb[0]), hexdec($rgb[1]), hexdec($rgb[2]));
    return $instance;
  }

  public function isHex($color) {
    return ctype_xdigit($color);
  }

  public function isRGB($color) {
    return is_array($color);
  }

  public function getRGB() {
    return array($this->red, $this->green, $this->blue);
  }

  public function getHex() {
    $hexRed = dechex($this->red);
    $hexGreen = dechex($this->green);
    $hexBlue = dechex($this->blue);
    return strtoupper($hexRed . $hexGreen . $hexGreen);
  }

  public function incrementRGB(array $rgb) {
    $result = array();
    $result[0] = $this->incrementColor('red', $rgb[0]);
    $result[1] = $this->incrementColor('green', $rgb[1]);
    $result[2] = $this->incrementColor('blue', $rgb[2]);
    return $result;
  }

  private function incrementColor($color, $quantity) {
    $result = $this->$color + $quantity;
    return $result;
  }
}