lovely, we have the Windsor and Newton Iridescent Watercolour Medium back in stock HERE. I've just been having a quick look around the web and our price is by far the best currently at £5.00 for a 75ml bottle - it lasts ages too.
Perfect for adding to cheap watercolours to give you a Cosmic Shimmer or Twinkling H20 effect for anyone on a tight budget (let's face it that's most of us at the moment!). There are many Youtube videos using it, but this one is very good.
I also use it over other paints and even Copic colouring for a really blinging experience.