[Steely Tom]

Everyone else did, at the time.

I guess it's a matter of opinion. I always thought "psychedelic" art looked more like this:

[VietVet876]

Real 200 octane acid trippin’ psychedelic art is epitomized by M.C. Escher.

[Albion Wilde]

There was no one image that was called psychedelic. Psychedelic art was an entire genre derived from the distortions of anything in the field of vision that people experienced when taking mind-altering recreational drugs such as LSD or mescaline, which were in popular use with hipsters when Peter Max started his career.

Psychedelic art was characterized by bright colors, often primary colors and/or fluorescent (Day-Glo) colors, and flat, simplified, cartoon-like images that appeared to float in the air, have distortions or exaggerated features such as lettering that appears to melt, people with legs much bigger than the rest of the body, depiction of whirling colors or shapes, and/or a mandala.

Use of the rainbow or spectrum effect was part of what people claimed to see around the edges of objects or people while taking those drugs, as well as other hallucinations. The spectrum of colors was only later used to signify homosexual political identification. In Peter Max's early work and that of other psychedelic artists, it was drug-imagery related.