Copied from Tv Tropes
"Well, several of Marx's attacks (raining seeds, ice balls, the mouth laser) are similar to attacks used by DJ Octavio, the Final Boss of Hero Mode in both Splatoon games, and they share a purple colour scheme as well as Flight (well, the DJ's mech flies). Also, Inkling's ink resembles paint; in Super Star Ultra, Paint was an ability Marx Soul could provide Kirby with."

I just thought because the ink-crediable journey had a huge color motif to it I thought Marx and his colorful design was some kind of tribute to it.