Indigo: Seventh Colour or Odd One Out?

Seven planets, seven notes in the Western major scale, so of course seven colours in the rainbow. Isaac Newton even felt that there were similar intervals between colours and notes. (You can hear the tones and see their associated colours on YouTube.) Presumably indigo fitted in well between blue and purple, but most modern scientists who study light consider the spectrum to consist of six colours, the primaries and their complementary colours.

Indigo is a useful term, however, for those colours that hover between blue and purple, inky and somber as they are. Some have suggested, in fact, that indigo is the colour of blue ink. The colour of new Levi’s, indigo means jeans for most people. It balances across the purple line from magenta. If magenta is in-your-face Lady Gaga, indigo is like Lou Reed – unflamboyantly cool.

The dye we call indigo comes from a group of plants all grouped into the genus Indigofera  (meaning they make indigo). Indigofera tinctoria grows in Asia, while in Central and South America indigofera suffruticosa is used. Unfortunately, most indigo plants are not only unshowy, but don’t have blue flowers. So much for the daydream of a brooding, dark blue flower that could produce a similar pigment.

The place where the dye and flowers intersect is perhaps in their rarity value: blue is an uncommon colour in nature, and both flowers and dyes are all the more prized for it. Indigo dye has been big business for at least 2 000 years. It was first cultivated in India, a fact reflected in the name. When the blue dye came West to Greece, they called it  indikón (ινδικόν, Indian).

The Romans took the name and Latinized it to indicum, which passed into Italian, and from there to English. The Oxford English Dictionary says the colour name indigo entered English in the mid-sixteenth century, from Portuguese índigo.

Indigo palette.

Indigo palette.

It has been suggested that Newton may have been influenced by the East Indian Company, which was bringing indigo dye back to Britain, where it was supplanting the native dye plant, woad. Both are sources of indigo dye, but woad has a much weaker concentration, and not surprisingly it was pushed out by the new colour from India.According to Ian Howard at Woad-inc, where they grow their own: “When tropical indigo (indigofera tintoria) began to be imported from Asia in the 16th century, woad could not compete in Quality or more importantly Quantity and the industry went into decline.”

The importing of indigo was extremely unpopular with dyers and woad-growers. Both France and Germany had passed laws forbidding indigo importation, but Europe couldn’t keep out the better dyes forever. They did give it a good try, however, with several countries, including France, banning its importation, and an international union of Woadites, mainly dyers, banding together to keep out indigo.

Newton may very well have been aware of the new colour, and the controversy surrounding it. He wrote his treatise on optics in 1660, by which time the controversy over woad vs. indigo had been running for about 100 years, with the East Indian Company just the latest salvo. (A more mundane explanation of his seven-colour system is that he had extremely poor eyesight, and it’s hard to distinguish the bluer colours of a rainbow anyway.)

When I think of indigo flowers, the first thing that I think of is monkshood, aconitum. Their dark, brooding blue seems appropriate for a plant so deadly poisonous.  If true blue is a rare colour in flowers, dark blue is even more unusual. There aren’t all that many that are a real, dusky blue. Some of the darker agapanthus, and the aquilegia ‘Blue Barlow’, along with salvias like ‘Caradonna’, spring to mind.

The advantage that monkshood has is that it started out dark, rather than being improved by breeders. Jacob’s Ladder or Polemonium caeruleum, is another naturally dark-blue flower, which echoes the shape of the monkshood while being much shorter. For height there are dark delphiniums, and clematis like ‘Prince Charles’ or ‘The President’. There is a herbaceous clematis species, clematis durandii, which has flowers the colour of slightly faded denim, a good echo of the monkshood.

clematis durandii

Clematis durandii. Photo by Simon Ross.

As for what to plant with your dark-blue flowers, you can go with the neighbours or you can go for opposites. As you’ve probably guessed from my other articles, I went the indigo – purple – magenta route. I used to have a stand of magenta phlox (no name, just a mutt) with the monkshood “Spark’s Variety” next to it. Behind them, on wires along the fence, grew the clematisNiobe” (rich reddish-purple) and “The President” (purple blue).


Clematis ‘Niobe”. Photo by Simon Ross.

Or, for more contrast, you can go right across the colour wheel to yellow–orange. So, think brassy yellow, the sort that tasteful people abominate. (They really do use words like that.)


Achillea “Terracotta” from the Earl May catalogue.

You could add rudbeckias, yellow achilleas, and either solidago (goldenrod) or golden heleniums for height. Kniphofias and daylilies would work too, the brassier the better. The grasses carex elata (Bowles’ golden sedge) and milium effusum (Bowles’ golden grass) could be used as well. Such a scheme would resonate well with what are called Venetian or Titan’s colours, most of which involve blues, golds and orange-reds.

Monkshoods, cooling the medium.

Monkshoods, cooling the medium.

For a shady spot, or a calm one, you could go for a more harmonious look. A very cooling scheme would involve some whites, to light it up, and blue-greens, maybe in the form of hostas, to calm it down. It would be entirely suitable for a colour that apparently symbolizes spirituality and the, and in Hindu symbolism is the colour of the sixth chakra, the third eye. From all this could come a very inspirational garden indeed.

PS – This is a reblog of a post from my MIssing My Garden blog. I want to do some pieces on colour for this section of my blog, and this piece and another on magenta will kick it off. This piece is also a cool example of how you can learn new stuff through blogging – I knew nothing about the upheaveal over indigo before I wrote this piece.
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