Esta es la funcion que expone la informaciondel error... solo basta que identifiques los colores usados, nose... la letra?? y los cambies a tu antojo.... yo no se de tus gustos
Código PHP:
function error($msg) {
$width = strlen($msg)*8;
$width = ($width < 370) ? 370 : $width;
$url = (strlen($this->url) > 45) ? substr($this->url,0,33)."...".substr($this->url,-10) : $this->url;
$this->rw = $width;
$this->rh = 145;
$this->source = @imagecreate ($this->rw, $this->rh);
$background_color = imagecolorallocate ($this->source, 230, 230, 230);
$text_color_red = imagecolorallocate ($this->source, 255,0,0);
$text_color_black = imagecolorallocate ($this->source, 50, 50, 50);
$text_color_green = imagecolorallocate ($this->source, 0, 150, 0);
$text_color_blue = imagecolorallocate ($this->source, 0, 0, 150);
imagestring ($this->source, 5, 19, 20, "ERROR: LA IMAGEN NO PUDO SER CARGADA", $text_color_red);
imagestring ($this->source, 3, 16, 50, "Hubo un error procesando la imagen:", $text_color_black);
imagestring ($this->source, 3, 16, 65, $url, $text_color_green);
imagestring ($this->source, 3, 16, 95, $msg, $text_color_blue);
imagestring ($this->source, 2, 10, 125, sprintf(base64_decode("UEhQSW1hZ2VuIChjKSVzIC0gQnkgTXlPa3JhbQ=="),date('Y')),$text_color_black);
$this->error = true;
return true;
}