In the Technical Art Questions Top 40, this query must have spent more time at No. 1 than any other. And it's a fair question, since one will see you buying a relatively inexpensive paint, and the other could see you spending two or three times as much.
So why is this? (Cue shimmering TV trope visual effect). If you roll back the clock to when pigment production was in its infancy, one of the qualities sought after by those early manufacturers was permanence. Now, 'infancy' could mean as far back as when our ancestors, grief-stricken by the death of a relative or pretty chuffed at the successfuly bludgeoning of an ill-tempered elk, first tried to immortalise that event on a cave wall, using those colours available to them. It's hard to believe that the earliest of artists didn't try a bit of everything, employing bog-standard trial-and-error to ascertain which of those 'found pigments' didn't flake off, didn't wash away, and didn't fade too quickly. We know from archaeological evidence that ochres, siennas and umbers - naturally-occurring iron compounds - stand the test of time; they still exist today, thousands of years later, paying testament to the ingenuity of the artists who painted with them.
Thanks to artistic pioneers, whether they be from the Palaeolithic or the Rennaisance, we now have at our disposal knowledge of thousands of pigments, most of them selected for vibrancy and permanence. The flipside of this is that, owing to the limitations of the time, safety hasn't been much of a consideration, and substances which are now well-known for their toxicity were commonly used in pigments. Heavy metals such as cadmium, cobalt and lead all found their way into the mix, with lead eventually being replaced wtih the far safer titanium dioxide.
The search for safer and, in some cases, cheaper pigments has lead to an increase in the synthesis of alternative compounds. These are what are used in 'hues'. Cadmium red hue does not contain any cadmium - it is simply 'the hue of' cadmium. The 'hue' indicates that the paint does not comprise the genuine pigments found in the non-hue product. An alternative, nearest colour, pigment, is used, or a blend of pigments, in order to create the colour.
Comparisons of genuine-pigment colours and their hues: (l-r) Cadmium red vs. cadmium red hue; cobalt blue vs. cobalt blue hue; cadmium orange vs. cadmium orange hue. (Copyright © 2016 Granthams Ltd.)
Artist quality paints (and variously Artists' Quality and Artist's Quality), i.e., those which contain genuine pigments rather than hues, are generally preferred by artists from the point of view that their constituent pigments do not fade badly over time. Hues may not replicate the vibrancy of their genuine counterparts, and they may be prone to fading in sunlight. Another issue which some artists experience is that the blended substitue pigments may separate during drying, resulting in a slight colour shift. Fading and colour shifts are not critical for everyone; the less expensive alternatives are often nicknamed 'Student Quality', in recognition of their popularity with those in education, for whom budget is another consideration.
As always, it comes down to preference. Jobbing illustrators who digitally scan their work soon after completion may not be too concerned with a pigment fading over time. Even professional artists and illustrators may have budgetary constraints. It's entirely possible that future hues will be closer than ever to their genuine counterparts, and it's also possible that some genuine pigments may be banned, as nearly happened to cadmium last year.
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