Peter Max and Heinz Edelmann. Edelmann was the art director on Yellow Submarine.
My first example of the Max / Edelmann style of illustration is the board game Payday, from 1975. Lame, I know, but I wasn't even born when Yellow Submarine came out. So, yeah, when I wind back the mental VCR to my earliest impression of 1970's cosmic art style, I get this on the screen:
OnTV and had a go at it himself.