One of my recent oil paint purchases was a tube of Winsor & Newton Indigo.

I wasn't quite sure what I expected, but I knew from their literature and website that the paint would be a mixture of blue and black pigments.

The original indigo was named after the blue dye pigment obtained from the plant Indigofera Tinctoria. The cut plant was soaked and fermented in large vats, and its dark precipitate skimmed, strained, pressed, and dried into cakes of indigo pigment. 

Indigo, several hundred years ago, was better than the other blue pigments that painters had at their disposal, which, though they produced brighter blues, had their drawbacks. Ultramarine was expensive, as was greenish blue azurite. While smalt and blue verditer were cheaper, both offered limited tinting strength and hiding power. Both azurite and smalt could only be used when coarsely ground, which made paint of these pigments hard to handle in oil.

In comparison, indigo had better covering power and tinting strength, and was used in easel painting from the middle ages onwards. Sadly, it sometimes tended to fade rapidly if exposed to sunlight, as this interesting dissertation by M.H. van Eikema Hommes reveals. If you're determined to use genuine indigo, and you've a mind to grind your own paint, you can still buy it online.

Sir Isaac Newton added indigo to his spectrum, declaring it to lie between blue and violet, in an effort to maintain a conceptual connection between the colours of the spectrum and the seven notes of the musical scale. Modern colour scientists, along with the rest of us who have eyes, are somewhat sceptical that Newton's indigo exists at all, and generally call any wavelength in the 450nM region violet.

Indigo, then, is not so much a colour as a blurry notion of a colour. I have a dim memory of reading Graham Joyce's novel of the same name, which describes seeing indigo in the evening sky, through the skylight of an atrium in a particular building in Chicago, which seems to be an awful lot of trouble to go to. Just to add to the merry confusion that is colour naming, there are new varieties of indigo, each with their own definition. Electric indigo, for example, is the name given to a particular shade, which has the following painting package colour codes:

#6F00FF in hex
(111,0,255) in sRGB
(57,100,0,0) in CMYK

Winsor & Newton Indigo doesn't look like that, and it won't fade like the old indigo pigment. Made from a Pthalo blue, Ultramarine blue, and Carbon black pigments, it looks, obviously, like a blueish black. Mixed with white, it gives a range of lovely blue greys which work well on snow scenes, and one of which was the precise shade I saw from outside my back door just half an hour ago, in the swollen underside of a raincloud. 

It's a very nice colour. I'm going to be actively looking for excuses to use it.