Back in the '60s, when boomers began to experiment with drugs, particularly LSD, the effect was labeled "psychedelic" — and the altered perception colored the psychedelic posters used to promote concerts. A half-century later, those posters are considered art — and the Denver Art Museum has become a major collector of them, as revealed by The Psychedelic Experience, a super-popular blockbuster last summer. The show was put together by AIGA graphics curator Darrin Alfred, drawing from the first-class collection of material assembled by Boulderite David Tippit, and it appealed to both graphic specialists and old hippies — a veritable poster child for an exhibit that was both accessible and artistically impressive.