Vincent van Gogh's fading colours

This is in response to Sylvia.  There is, I have discovered, an awful lot of old cobblers on various arty websites!  For good information on the problems with van Gogh's yellows (and reds) I had to consult an engineering website (the name of which I've instantly forgotten, but Mr Google will find it). Both van Gogh's yellows and bright reds have faded; the reds have faded in his Irises painting, turning the purple he intended to blue (it's still a lovely painting though).  It's also faded on some of his most famous paintings, eg the interior, with bed and chair, turning the pink floor brown, and the purple walls blue.  The problem here was red lake pigments containing cochineal - he knew it was fugitive, but: there wasn't anything else available in quite the colour he wanted.   I wonder if he ever used Rose Madder, which would have been a rather safer choice.  But more crimson than the red he was after.  Yellow - a favourite colour of his - was problematic because of a) the experimental nature of the paints; b) adverse reactions to later varnishing (van Gogh rarely varnished his own work, but conservators sometimes did, eg when they noticed cracking or flaking).   Van Gogh used Chrome Yellow, which is well known to fade in its early manufacture (it can be got today: I wouldn't, though); and, when he could afford it, Cadmium Yellow - certain versions of which can also fade, turn brown, generally degrade.  It's believed to be a good, permanent paint now.  I found a website which decries it - and makes a stupid comment about van Gogh's use of it, as though Chrome Yellow was better: the truth there is that Cadmium Yellow is disliked by some because it's toxic; but then, so is/was genuine Chrome Yellow.  Lots of things are toxic, we just learn to handle them safely.   There's a long history of paint-makers trying to improve the permanence of pigments: when I began painting, there were many paints which weren't know to be fugitive - Alizarin Crimson in particular - and back in the 60s and 70s we were advised not to use French Ultramarine with Flake White: chance would be a fine thing now, given how difficult to get FW can be, but this wouldn't be a problem today thanks to changes in the composition of one or the other, and possibly both.   Paints to be wary of today in oil - not always in other media - are the aforementioned Alizarin, and Zinc white: the first fades, the second causes cracking and delamination (or can cause it: especially on flexible surfaces, because it dries both hard and brittle, and over time can react with other pigments to make metallic soaps, which attack the paint film).  You also need to have a care with Titanium White - the pure paint, PW6, is fine - but often it's mixed with PW4, Zinc White - and we're not always told.   PW4 also features in nearly every brand of Naples Yellow Hue - not in the real (and expensive) Naples Yellow.   In short .... Vincent had problems, so do we all these years later.  When dealing with light, bright colours, particularly, we still need to be very careful with the pigments we choose, and it does pay to learn about them.  Van Gogh was apparently quite a student of paint-making, but like most painters then and now, he took risks - hoping the colour wouldn't fade, resigning himself to the fact that some of  it probably would.