Let's talk about a few other costumes from the rough draft, namely the Imperials:
The Emperor presumably wore a black military uniform, like his counterpart in Dune, Padishah Emperor Shaddam IV (who invariably was seen wearing a "gray Sardaukar uniform with silver and gold trim").
However, since the majority of Imperial officers would also wear black and gray uniforms, the Emperor would need something distinctive to set him apart on celluloid. Probably he would have worn a cloak or cape of some sort--likely gold in color.
As Emperor in Dune Messiah, Paul Atreides wears golden robes on formal occasions. This idea also shows up in Leigh Brackett's first draft script of ESB, where the Emperor is "draped and hooded in cloth-of-gold."
This costume would give the rough draft's Emperor an extraordinary resemblance to Emperor Ming in the 1940 serial Flash Gordon Conquers the Universe. In this serial Ming wears an elaborate military uniform with a cape (in the earlier Flash Gordon serials he wore relatively simple robes instead).
One thing the Dark Horse comic adaptation got right is that General Darth Vader would likely have had some sort of facial scar. This would echo the dueling scars frequently seen on fictional Nazi officers (a relic of real-life Prussian military culture).
Colonel Dietrich sports just such a dueling scar in Raiders of the Lost Ark; he serves as a brutal henchman and foil to the more refined Belloq, just as General Vader would have been to Crispin Hoedaack.
Although the rough draft describes Prince Valorum as wearing a "fascist black-and-chrome uniform," given his status as a Black Knight of the Sith, he presumably would have worn black robes, like the other Sith knight seen earlier in the script (and Darth Maul in TPM).
The black robes are a dark mirror of the earth-toned robes sported by the Jedi: a reflection of the Empire's perversion of a noble order of warriors.
Darth Maul's skin--mostly black, with red tattoos--reflects the idea Lucas probably had to cast Prince Valorum as an African-American.
Later on, Valorum is demoted in rank, and is seen wearing the uniform of a common stormtrooper. (At this stage the stormtroopers likely had black armor.)
I suspect that nobody would have been able to resist the temptation to add a black cape to Valorum's stormtrooper armor, just to make him stand out on screen.
(This is actually a Ralph McQuarrie concept of a Sith Lord from the third draft. But you get the idea.)
In the final scene at the end of the film, Valorum would undoubtedly have been shown wearing white robes, as a sign of his change of allegiance.