Only a handful of artists can be described as pivotal. Michelangelo, Velásquez, Manet and Duchamp spring to mind – and so does Jasper Johns. Inspired by a dream, the Georgia-born artist produced his first collage-painting of the Stars and Stripes in 1954-1955. From then on – in synergy with his friend Robert Rauschenberg – Johns’ use of popular iconography wrenched the landscape of American painting away from the emotive terrain of abstract expressionism and set it on course for the cooler, more cerebral movements of pop art, minimalism and conceptualism.

Fifty years after that first flag, Johns’ oeuvre has been the subject of myriad critical dissections. The artist, however, always insists that his work’s meanings are multiple, elusive and fluctuating; visitors to his latest show would do well to remember his advice.

Entitled Gray, the exhibition gathers 120 paintings, drawings, sculptures, reliefs and prints. Spanning Johns’ career, what unites them is the colour of the title. But is their shared hue significant in itself? After all, Johns is prolific; even as he was producing these grisaille pieces, he was simultaneously creating hundreds of other artworks in colour.

It turns out that Johns worked more often in grey than in any other single hue. Yet in a recent interview, he admitted that he thought of his favourite shade mostly “as an absence of colour”. It seems that, at the beginning of his career at least, Johns perceived grey as a negative charge: a neutral, inert non-colour, devoid of symbolic power. Of his early works, he says that “grey drained the work of the excitement that colour afforded”. For many painters, these attributes would make it a shade to avoid. (Delacroix, a supreme colourist, derided grey as the “enemy of painting”.) But in the days when Johns was determined to transcend the nervy, chromatic ecstasies of abstract expressionism, it was the perfect medium.

Johns’ anti-expressionism is superbly orchestrated in the opening room, which is devoted to two 1959 canvases: “False Start” and “Jubilee”. The former is saturated in thickets of red, blue and yellow but this typically abstract expressionist style is undermined – with Wittgenstein-like intuition about the lacuna between language and meaning – by colour names stencilled in non-corresponding hues. “Jubilee” goes a step further, displaying the identical sunburst pattern in grisaille. A lifeless, exhausted ghost of its pendant, it acts as a chilling, ironic snub to the passionate, colour-charged intensity of Pollock, De Kooning and friends.

Johns’ jubilation at the symbolic assassination of his forebears is indicative of an innate iconoclasm. “When I look at other artists I automatically think: ‘That’s what I should avoid in my own painting’,” he has said.

After a miserable South Carolina childhood, Johns arrived in New York in 1948 at 18 years old having “never really seen paintings”. Following a brief stint at the Parsons School of Design, he left to pursue his own path. Encouraged by Rauschenberg, whose “combine” paintings traversed similar terrain, Johns explored the idea of paintings as formal objects rather than as images designed to express meaning. The use of grey, he discovered, allowed “the object nature of the materials to come forward”.

A clutch of canvases from the late 1950s illustrates this perception. Painted in cold, slatey tones, their surfaces have been tampered and tormented: cut into rectangles, inlaid with a fake drawer, hung with a coathanger.

Next come a skein of exquisitely nuanced calibrations on Johns’ trademark motifs – flags, targets, maps, stencilled letters and numbers. The soothing, repetitive rhythm of these works makes them forerunners of the dispassionate grids of minimalist Agnes Martin and the sequential mind games of Sol LeWitt. Yet Johns’ love affair with texture puts them on another plane. Daubed in sumptuous, clotted, pewter-grey flakes of encaustic, moulded in scintillant, cracked-silver fragments of sculptmetal, crayoned in waxy black or lithographed on to mottled paper, his signature icons surrender to an abstraction that is more sensuous than cerebral.

Other pictures are devoid of such opulence. In “Good Time Charley” (1961) and “Fool’s House” (1962), for instance, tools from the artist’s workshop – beakers, rulers, a broomstick – are attached to canvases whose surfaces, ruptured by bare patches, are tinged with cold blacks and whites and traced with arc-like ghosts of his target symbols. Ominous, transgressive, anti-monumental, they explode the myth of artist-as-hero, reducing him to the level of artisan or carpenter.

More painterly are his cross-hatched canvases. Johns spotted this herringbone pattern, which swathed his surfaces in the late 1970s and 1980s, on the paintwork of a car and instantly fell for its “possibility of a complete lack of meaning”. Notable here is “Between the Clock and the Bed” (1982-1983), inspired by Edvard Munch’s eponymous self-portrait. Blanketed with a thatched weave in black, white and grey over minute shavings of colour, the triptych vibrates with the mobile, brooding energy of a night-time blizzard.

Such symbolic imagery is the unexpected beauty of this show. For as Johns’ vision evolves over the decades, his greys take on an evocative – dare one say it, expressive – tendency. In “Winter” (1986), for example, he leans on the colour’s elemental associations to create an ironic, existential drama, complete with fat snowflakes, childlike snowman and a shadowy, silhouetted figure.

More lyrical and serene is the recent “Catenary” series. These paintings, drawings and prints, their surfaces coloured in nuanced grey monochromes, are distinguished by a loop of string suspended from either side of the canvas. The star is “Near the Lagoon”, (2002-2003). Inspired by Degas’ recuperation of a fragmented painting by Manet, the canvas is a Russian Doll of hidden panels, hinges and underpainting, yet it resembles a lake at twilight, its mirror-still surface traced with arcs and lines of shadow.

Throughout his career, Johns’ works have referenced poets, including Tennyson, Hart Crane and Beckett. “Bridge” (1997), the first Catenary painting, was inspired by Hart Crane’s ode to Brooklyn Bridge. At first, Johns’ work – the string swooping across the blackboard-like expanse framed by Picassoesque imagery – is intimidatingly hermetic. Yet Crane’s poem brings revelation. Comparing the bridge’s “inviolate curve” to a “rip-tooth of the sky’s acetylene”, it finishes by begging the bridge to “descend/And of the curveship lend a myth to God.” If Johns can nudge us towards such magnificent imagery, perhaps neither he – nor his greys – are enemies of expression after all.

‘Gray’ continues at at the Metropolitan Museum of Art, New York until May 4 www.metmuseum.org