Was Vegeta's design altered mid series?
From what I can find, there was no official explanation given for the color change. A couple of fan theories, including this one, are basically that the coloring was done before the mangaka announced the actual hair color, and that they fixed it once the color was announced. This is corroborated by the Wikia trivia for the episode, which states:


  This episode marks the debut appearance of Vegeta and Nappa in the anime. They are shown in colors quite different from their later ones, especially Vegeta. This is because Toei guessed their colors, as the corresponding Manga volume was not in color. Vegeta's case is corrected in the Dragon Ball Kai version of this episode and in the Dragon Ball Full Color Manga version as well. Nappa's was not, though it is possible he owns more than one suit of Battle Armor.


In-universe, his hair didn't change color, as he stated


  that a pure-blooded Saiyan's hair does not change from the day they are born, aside from Super Saiyan transformations (Wikia)