I've been looking for some more information to build on yesterday's question, "Why is denim blue?". As always, Wikipedia has provided some really useful information on the colour Indigo, and I created the image on the left to show what they mean.
"Like many other colours (orange and violet are the best-known), indigo gets its name from an object in the natural world—the plant named indigo once used for dyeing cloth. The colour electric indigo is an approximation of spectrum indigo. This is the brightest colour indigo that can be approximated on a computer screen—it is the colour between the web colour blue and the colour electric violet.
The web colour blue violet or deep indigo is a shade of indigo brighter than pigment indigo but not as bright as electric indigo. The colour pigment indigo is equivalent to the web colour indigo and approximates the colour indigo that is usually reproduced in pigments and coloured pencils.
The colour of indigo dye is a different colour than either spectrum indigo or pigment indigo. This is the actual colour of the dye from the indigo plant when swatched onto raw fabric. A vat full of this dye is a darker colour, approximating the web colour Midnight Blue.
When specifying the colour indigo, it is necessary to indicate which particular one of these four major shades of indigo you are referring to."
There's a lot more information on all four the types of indigo colours on Wikipedia. It'll definitely make me think the next time I say "blue" jeans.