Ed Balls may have confused pink with indigo when discussing the rainbow on the cover of the Children's Plan booklet last week, but his department was correct to omit indigo (TES, January 11). Most physicists now accept that although, as any artist will tell you, there is a colour called indigo, it is not present in the rainbow. Sir Isaac Newton inserted it into the rainbow only because his own religious beliefs required it to have seven colours.
You can easily check this for yourself by looking at a rainbow or spectrum. The only rule is that it must be a real rainbow or spectrum: because of the way colours are produced by computer monitors and television screens and in print, a picture of a rainbow will not do.
Barry Gray, Private tutor, Gillingham, Kent.