I had a feeling that these counterless letters filled with rainbow patterns might be typographic, i.e. prefabricated, as opposed to custom drawn (note to self: when in doubt, it is always a font). I couldn’t quite put my finger on it, though.
Luckily, Flickr user jch02140identified the typeface as Hollenstein’s Indigo, a rather obscure design from the phototype era. He also pointed out the similarity to Rainbow Bass by Saul Bass.