Cadmium provides brilliants shades of yellows, orange and red paints. Before of cadmium-based pigments, painters hadn’t such permanent reds. Cadmium colours are also vibrants, with a high chroma, and have a strong tinting power.
Unfortunately, cadmium is a toxic metal. With proper care the artist can avoid contact while handling materials, but a safer option would be to choose other pigments. Cadmium is a cumulative toxin, that can be absorbed through inhalation of its dust (a risk to whom make their own paints), by ingestion or through skin. Even that dermal absorption is not a significant route of cadmium entry, it happens. And, even very low levels of this metal may have adverse effects in our body (especially in the kidneys).
Cadmium red is cadmium sulfoselenide, and as a pigment is known as PR 108.
A “perfect substitute” would have all the same properties of the pigment, what, at least presently, is not possible. But there are some good alternatives.
So, for the cadmium red (PR 108), I’ll highlight these: PR 254, PR 255, PR 112, PR 170, PR 149. For what I’ve seen so far the first two seems to be better as “substitutes”, due to its lightfastness (if this is a main concern).
PR 254 and PR 255: Pyrrole reds. Excelent lightfastness (888 at the BWS). They have high chroma, and are highly staining. The PR 255 is slightly yellower than the PR 254. They are less opaques than the cadmium reds. Examples – PR 254: Maimeri Puro Sandal Red, Rembrandt Permanent Red Deep. PR 255: Blockx Pyrrolo Red, Rembrandt Permanent Red Medium.
PR 112: Naphtol red. It’s a intense bright yellowish red. Heavily staining. Less opaque than cadmium (but I think that it is a bit more opaque than the pyrroles – to be confirmed). It has been rated as having excellent lightfastness under ASTM testing, but other testing methods have demonstrated some tendancy for the color to fade with exposure to strong UV light, and its is BWS 7-8;6-7;5. M Graham Naphtol Red, for example, has its lightfastness labeled as “Lightfast Rating Very Good – LF II”. Examples: Williamsburg Fanchon Red (ASTM II), Old Holland Scheveningen Red Medium.
PR 170: A Napthtol red. Its lightfastness may not be reliable (ASTM I-III). I found this interesting info on Dickblick “The lightfastness and weatherfastness of Pigment PR170 varies, depending on the application and the crystaline form. The opaque form (F3RK) has very good lightfastness, and is more weather resistant.” Sennelier Cadmium Red Medium Hue is rated as ASTM II (good lightfastness). Examples: Sennelier Cadmium Medium Hue, Michael Harding Scarlet Lake.
PR 149: Perylene red. It has good lightfastness and permanence. It may darkens (with exposure to sunlight). But Winsor and Newton Artists’ Winsor Red Deep is labeled as “Lightfastness (ASTM) I”. It’s a semitransparent color. Examples: Michael Harding Crimson Lake, Rembrandt Scarlet, Holbein Artists’ Perylene Red.
Although descriptions of pigments technical features are hightly useful, they can’t replace the experience of using a particular pigment or oil paint.
UPDATE: I’ve bought a tube of Rembrandt “Permanent Red Medium” (#377), made with PR255. The color is pretty much similar to the Cadmium Red Medium of the same brand, maybe slightly (really slightly) warmer. The great differences of this pyrrole to this cadmium: its transparency and weaker tinting strength, compared with a cadmium (although it has a good tinting strengh).