The Well-Tempered Clavier

Drive-to-work question, while listening to The Well-Tempered Clavier.

Q. Why is the eighth Prelude/Fugue specified as "E flat minor / D sharp minor" ?

I appreciate the two keys are equivalent but Bach must have scored it with either six flats or six sharps. Mustn't he? Anyone know which?

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A. Apparently the Prelude is scored in E flat minor, but the Fugue is scored in D sharp minor.
This is used as an argument by some for the hypothesis that Bach wrote for equal temperament, not just for well temperament, but this argument can't be sustained, in my opinion. If a keyboard sounds in tune for D # minor, by whatever means, it must sound just as in tune for E flat minor.

Well-temperament (which allows all keys to be playable, but includes some subtly different intervals between notes) would make keys sound different in character and this could perhaps be connected with the impressions of key-colour that some enthusiasts have recorded. (E.g. Christian Schubart on D sharp minor: Feelings of the anxiety of the soul's deepest distress, of brooding despair, of blackest depresssion, of the most gloomy condition of the soul. Every fear, every hesitation of the shuddering heart, breathes out of horrible D# minor. If ghosts could speak, their speech would approximate this key.)

Personally I don't think this has a lot to do with tuning, it has everything to do with mass psychology and there are other key-characteristics that could equally well provoke the imagination to such flights.

For example: familiarity and (relative) unfamiliarity. The fact that that first piano piece you learned was in C major, not in B major. The fact that the tunings of open strings on violin, guitar, etc direct the beginner toward the keys with a small number of sharps. And then the literature that follows from this, and the body of associations that develop.

And after all, well-temperament would only have applied to keyboard instruments, not to melodies played on the strings. [Except, of course, when they're trying to stay in tune with a keyboard instrument. It is said that classical musicians are now so habituated to equal temperament that they play in it even when they don't need to.]

Finally, if well-temperament were an important factor then people would no longer have ideas of key-colour, but of course we do; not experiencing the historically-bound romantic notions of Schubart's vehement fancies - very much the product of their era - but certainly recognizing the softness of A minor(Schumann's piano concerto) or the solidity of E (Chopin's prelude) - pick your own associations. In my own case - not having perfect pitch - I am certain I only experience key-colour when I know exactly what key I'm listening to.

[This post is really a belated follow-up to this earlier one on the 24-keys prelude form.]

Scriabin experienced key-colour more literally. His synaesthesic impressions appear to match the visible spectrum sequence VIBGYOR as you go through the cycle of fifths. For him, his own cycle of preludes would begin red in C major, then turn orange (G), yellow, green, blue, deep-blue (F sharp / G flat), purple, violet, flesh, rose/steel, deep-red (F) ....
So the first bit follows the rainbow, and the second, briefer bit (from A flat to C) tones from violet back to red along a different path.

Info from here.