Me and my friends chalked a person's Ki color up to genetics and forms (broly's base ki is green while goku or vegeta's differs from form to form) but actual attacks like the Kamehameha or Masenko are always blue or yellow.

Usually, the villains have this type of ki. This nature of ki can be changed if the user holds no evil intentions, as the case of Vegeta who eventually became a good person. The appearance of this type of ki is usually a purple or pink aura around the user, like Vegeta (during the Vegeta, Namek, and Frieza Sagas) and Frieza.

Vegeta's aura is originally purple on earth and then it becomes a pale blue on Namek. Piccolo's was blue but then becomes white after he is wished back. Frieza's goes from blue to red and then back to blue. But white colored auras are usually given to pure warriors. Vegeta is never pure so his stayed blue. I'm just trying to rationalize it a bit.

Most ki blasts during the saiyan arc of the full color manga were yellow, the heroes had yellow ki blasts and even Raditz, Nappa and Vegeta had yellow ki blasts despite being villans.

Cell retained the white, blue, and gold to show his relative similarities to the Saiyans. 18 was changes to pink to give her something unique to distinguish her from her brother. Vegeta's changed from purple to white to blue as a short-hand to the audience for his fluctuating allegiance.