I've had difficulty getting this function to return the number of unique colors.  Also, I wanted to be able to have an array with each $key value being the RGB of the pixel, and each $value value being the number of times that pixel occurs.  Basically, a frequency list.  For example, you would have "1 / 0 / 0" for red as a $key value, and "25" for the number of times that pixel color was in the image.  So, I wrote some code to do that, using a combination of readImageFile, getImageWidth, getImageHeight, getImagePixelColor, and a simple x/y parser, like so :
<?php
        $file_to_grab_with_location = "test.bmp";
    $imagick_type = new Imagick();
            
        $file_handle_for_viewing_image_file = fopen($file_to_grab_with_location, 'a+');
    $imagick_type->readImageFile($file_handle_for_viewing_image_file);
    
        $frequency_list_of_values = array();
    
        $image_resolution_width = $imagick_type->getImageWidth();
    $image_resolution_height = $imagick_type->getImageHeight();
        
    print("Image Resolution:  Width - $image_resolution_width / Height - $image_resolution_height<br><br>");
    
        for($y = 0; $y < $image_resolution_height; $y++)
    {
            for($x = 0; $x < $image_resolution_width; $x++)
        {
        
                $pixel_to_examine = $imagick_type->getImagePixelColor($x,$y);
            
            $pixel_to_examine_color_value_red = $pixel_to_examine->getColorValue(imagick::COLOR_RED);
            $pixel_to_examine_color_value_green = $pixel_to_examine->getColorValue(imagick::COLOR_GREEN);
            $pixel_to_examine_color_value_blue = $pixel_to_examine->getColorValue(imagick::COLOR_BLUE);
                $key_value =    $pixel_to_examine_color_value_red    .    " / "    .
                    $pixel_to_examine_color_value_green    .    " / "    .
                    $pixel_to_examine_color_value_blue            ;
    
                if(isset($frequency_list_of_values[$key_value]) == TRUE)
            {
                $temp = $frequency_list_of_values[$key_value];
                $temp++;
                $frequency_list_of_values[$key_value] = $temp;
            }
            else
            {
                $frequency_list_of_values[$key_value] = 1;
            }
        }
    }
                
        print("<pre>");
    print_r($frequency_list_of_values);
    print("</pre>");
?>