Yeah you can make the warning whatever color you want and also use a larger font site. Open the “eStore_cart.php” file and look for the following line:
$output .= '<br /><span id="pinfo" style="display: none; font-weight: bold; color: red;">'.ESTORE_QUANTITY_CHANGE_NOT_ALLOWED.'</span>';
See the bit of inline CSS there? change it to however you want the warning text to look like.