According to Empire's "making of" book, Once Upon a Galaxy, Boba Fett's character had originally emerged from that of Darth Vader:
"I wanted to develop an essentially evil, very frightening character," said Lucas of Vader. "He started as a kind of intergalactic bounty hunter, evolved into a grotesque knight, and as I got deeper into the knight ethos he became more a dark warrior than a mercenary... I split him up and it was from the early concept of Darth Vader as a bounty hunter that Boba Fett came."
Borne of Vader, Fett required an equally villainous look, but something a bit less conspicuous. Designing Fett's signature helmet, armor, and accoutrements fell primarily to Joe Johnston, who worked in tandem with Ralph McQuarrie to come up with the bounty hunter's distinctive look. Far removed from the black-on-black wardrobe of Vader, early concepts for Fett clad the bounty hunter in white, possibly a vestige of his "Super Trooper" origins. This all-white Fett was actually the first costume produced for the character, officially unveiled to Lucasfilm insiders in a screen test shot on June 28, 1978 (you can view part of that original screen test here).
In the 20-minute black and white video, sound designer Ben Burtt "hosts" Fett's reveal for Lucas and crew, describing the different weapons, functions, and characteristics of the costume (worn by Empire's assistant film editor Duwayne Dunham for the test). The somewhat amusing footage depicts Fett with a mocked-up laser rifle (which used a lightsaber hilt for the barrel) and a Star Wars beach towel doubling as the bounty hunter's tattered serape.
Even at this early stage, the plan was to give the costume a muted color scheme, visually placing the character somewhere between the rank-and-file stormtrooper and the fearsome Dark Lord of the Sith. "I painted Boba's outfit and tried to make it look like it was made of different pieces of armor," said Johnston in Star Wars: The Annotated Screenplays. "It was a symmetrical design, but I painted it in such a way that it looked like he had scavenged parts and had done some personalizing of his costume; he had little trophies hanging from his belt, and he had little braids of hair, almost like a collection of scalps."
Fett's new color scheme was at least partially revealed in "The Star Wars Holiday Special", which aired on November 17, 1978. Animators at Nelvana Studios simplified the costume's palette a bit by painting the character in various shades of blue and green (a scheme which was repeated for Fett's post-trilogy cameo in 1985's Droids cartoon series). The Holiday Special introduced Fett as a friend of the Rebels, but was soon found to be in secret collusion with Darth Vader. His true colors revealed (in more ways than one), Fett was now primed to be the next major villain introduced in the Star Wars sequel, The Empire Strikes Back.