My very well made theory

The color purple was revered in ancient Rome as a symbol of power and status, and was worn by emperors such as Marcus Aurelius, who used it to assert their authority. Marcus Aurelius was known for his wisdom and philosophy, and he compiled his personal reflections in a collection known as “Meditations.”

Looking at Waluigi’s color scheme, we can see that he is embracing the regal association of purple, aligning himself with the imagery of royalty and nobility. Perhaps Waluigi was once the emperor of the Mushroom Kingdom, but he was an unpopular ruler. Peach eventually took over, and Waluigi was forced to leave the throne, as he was an outcast from society.

In his despair, Waluigi was found by Wario, who took him under his wing and promised to help him exact his revenge against the Mushroom Kingdom. Instead, Wario and Waluigi formed an alliance, united in their desire to go against Mario and Luigi on the tennis court since waluigi felt that being the ruler of the mushroom kingdom was irrelevant now that he’s chosen luigi as his rival.(also, trying to reclaim the throne was 2 hard.)

Waluigi has always been jealous of Luigi’s fame and popularity. He feels that Luigi gets too much attention and credit, while he himself has always been overshadowed and disregarded. He has developed a deep resentment towards Luigi due to this, which manifests as a deep-seated hatred and desire to “one-up” him in any way possible. He can not handle the fact that Luigi often bests him. Some fans may see it as a reflection of Waluigi’s own insecurities.

Waluigi still wears purple as a reminder of his former status as an emperor, a symbol of his former power and the potential that he still holds although he gave up his goal of ruling over the mushroom kingdom to play tennis against his rival luigi.