The American flag is not what it appears to be. Sometimes it’s red, white and blue, but other times it’s green, black and orange. The same goes for a bull’s-eye. Instead of the usual black-on-white (or Target red), this one is yellow and blue against a crimson background, with the noses and mouths of four plaster-cast faces stationed above it.

These “symbols that the mind already knows” are a trademark of Jasper Johns, who maintains an elusiveness in life and art even as he approaches 90.

“He is probably the most celebrated living American artist of the moment, and people have been interested in what he’s been doing since the 1950s,” said Joan Rothfuss, guest curator for the exhibition “An Art of Changes: Jasper Johns Prints, 1960-2018,” opening this weekend at Walker Art Center.

With about 90 prints, the show encompasses nearly 60 years of work, from Johns’ early prints of familiar symbols to his more mysterious pieces of recent decades.

Organized chronologically, the exhibit shows how Johns returned again and again to certain symbols, even referencing earlier works as in the 1986 lithograph “Ventriloquist,” where viewers can see the green-and-black-striped flag he famously used for a Vietnam War protest.

In curating a Jasper Johns show, “you could do a lot of things,” said Rothfuss. “You could do a show about gray, which someone already did. ... I wanted to give people a way to see how he revises his motifs, returning to favorite ones decades later.”

This show, continuing through Sept. 20, is the second major survey of Johns’ work at the Walker, which owns every print the artist ever made — 414 in all.

What the mind knows

The subject matter of Johns’ prints looks familiar to viewers because it is.

The 1968 lithograph “Gray Alphabets” is a series of rows and columns of all 26 letters of the alphabet. His piece “0 through 9,” made on an embossed sheet of lead, layers the numbers in a mysterious way, compressing them so they begin to feel like an infinity sign.

What at first appears basic actually harbors deeper symbolism. Johns plays a game with the interpretation of signs and symbols, essentially a visual study of semiotics.

He likes to play optical tricks, too. The 1967-68 lithograph “Flags” appears to simply be two U.S. flags — one in orange, green and black, the other in shades of gray — displayed against a dark gray background.

Look harder, though, and you’ll see a tiny white dot in the middle of the colored flag, and a black dot in the grayed-out one. Stare at the white dot for 15 to 20 seconds. Then look at the black dot in the middle of the grayed-out flag to catch a hidden symbol.

The U.S. flag keeps showing up throughout the exhibition, all the way up to the 1998 etching “Flag on Orange.”

“One night I dreamed that I painted a large American flag,” Johns famously said, “and the next morning I got up and I went out and bought the materials to begin it.”

This desire to use familiar symbols early on was a strategic move on Johns’ part, noted Rothfuss: “He consciously tried to take [self-expression] out of his art, which was in contrast to people like Jackson Pollock, whose works were supposed to reveal his inner thoughts.”

In doing so, Johns helped usher in Pop art, erasing the division between fine art and mass culture. Savarin Coffee cans, pages ripped from newspapers, a crisscross line design he saw while driving around, and found objects all made their way into his work.

Things shifted in 1982, when he felt it was time to bring his emotions back into the work.

“In my early work, I tried to hide my personality, my psychological state, my emotions,” Johns candidly admitted in an oft-quoted explanation of his work. “I stuck to ,my guns for a while but eventually it seemed like a losing battle. Finally one must simply drop the reserve.”

That vulnerability is manifest in his more mysterious work of the past four decades. Johns’ looming shadow becomes a familiar figure, seen here in the four 1987 intaglio-on-paper works from “The Seasons.”

In the ambiguous 1988 print “Untitled,” three increasingly abstract portraits appear to be pinned onto a canvas filled with floating shapes. The portrait on the left references a famous optical illusion, “My Wife and My Mother-In-Law,” that appears to be either a young woman or an old one, depending on how you look at it. The middle portrait includes one of Picasso’s anomalously shaped faces. The last is based on a schizophrenic child’s drawing of a mother, her brown eyes located in opposite corners.

Johns’ most recent print, “Untitled” (2018), is both morbid and playful. A skeleton wearing a little top hat stands or lies in a coffin, holding a skull over its genital area.

What does the skeleton symbolize? The slang phrase “getting it through one’s thick skull” suggests a struggle to comprehend the mystery of death. Or maybe Johns has something cruder in mind (guess what “getting skull” means). In true Johns fashion, we may never know.