I thought I'd share a photo of a test of fugitiveness (light fastness) I've done this year - using art pens in a variety of colours.
One strip has been in my studio window for 6 weeks - so although it's had sunshine, it's only been winter light - the other tucked away (OK - lost in a pile of papers until today!)
Look at the purple and pinks, plus the palest grey - all my hard work on my calligraphy or illustration would have been for nothing if I used those colours.
It's an easy but informative exercise!