Lightfastness: Which Colours Are Still There in Fifty Years

The colour NAME on a tube means nothing — 'permanent rose' and 'crimson' are marketing, and two makers use them for different chemicals. The COLOUR INDEX number does mean something: PB29 is ultramarine, PR108 is cadmium red, PY35 is cadmium yellow, wherever you buy it. The lightfastness rating is usually ASTM I to V, or a star system. ASTM I is excellent, II is very good, and III and below should not be used for anything meant to last. A tube with no pigment index and no lightfastness rating is telling you something. Student ranges frequently substitute cheap fugitive pigments under traditional names, which is legitimate for practice work and not for anything you will still want in twenty years.

ORGANIC LAKE pigments — dyes precipitated onto a transparent substrate — are the classic fugitives. Madder lake, carmine from cochineal, and many bright modern pinks and violets fade because the dye molecule itself is broken apart by ultraviolet light. MINERAL pigments are generally far more stable, because there is nothing to break apart: an iron oxide is already the stable oxidised form. That is why earths, ochres, and the cadmiums and cobalts endure. There are specific exceptions worth knowing. Vermilion blackens. Lead white darkens in sulphurous air. Prussian blue fades in light and partly recovers in the dark, which is genuinely strange and well documented. Chrome yellow darkens, which is what happened to some of Van Gogh's sunflowers.

Paint a strip of every colour you use, cut it in half, and put one half in a south-facing window and the other in a closed drawer. Compare them after six months and after a year. That is a controlled experiment with a real control, and it settles the question for your paints in your climate better than any published table. It is the change-one-thing-at-a-time rung applied to time. Date the strips and photograph them at the start, because memory for colour is very poor. The half kept in the dark is the whole point: without it, a slow fade in both halves is invisible.

A thin wash or a glaze has far less pigment per unit area than a solid layer, so destroying a given fraction of it changes the colour far more. The same pigment can look permanent in a thick passage and fade visibly where it was used thinly. Watercolour is therefore the most vulnerable medium of all: thin films, no protective varnish, and traditionally full of lakes. That is why watercolours are shown under low light and stored in the dark, and why the ones that survive were painted with earths and mineral colours. Which turns lightfastness into a design decision rather than a purchasing one. If a colour must be thin and must last, it has to be a mineral pigment — and if the only pigment with that hue is fugitive, the honest answer is to change the picture.