I've just purchased a couple of bottles of Dr. Ph. Martin's Hydrus Fine Art Watercolours to try out for watercolour painting and woodblock printing. I couldn't find a single reference to their lightfastness on'th internet although the marketing blurb says that they're produced using pure pigment, not dye and that they are all permanent and lightfast... we'll see...
I'm going to run a test over the next six months to compare their lightfastness against Winsor & Newton Artist's Watercolour. I'll paint stripes of colour using red ,yellow and blue from both products and leave the paper partially exposed in a south-facing window.
Every two months I'll expose a little strip of the colours to the light, and after six months I'll have a colour chart of results ranging from unexposed, through two and four months to six months.
I hope the results will be useful to anyone else wondering about this product.
I'll post update images every two months.