Gouache Longevity

I have recently visited a client of mine in Brisbane.  She has a number of my artworks and purchased her first gouache painting from me over 20 years ago.  The painting is hanging in a room with large windows all around and it is always well lit with the most beautiful Queensland sunlight.  I was very pleased to note that the painting has not faded at all.   The gouache I used is Daler Rowney Designers Gouache and this has certainly demonstrated the quality and longevity of the paint.  Mind you, hanging work in bright sunlight is a challenge to the best works of art.