Love is just a four letter word – and it was a much ruder one when Robert Indiana, who has died aged 89, first thought up the work of art that was to define him and an entire decade. Indiana’s 1966 pop masterpiece Love originally said Fuck. Even after he changed it to the more heartwarming and universally acceptable word – he would spend the rest of his life turning it into sculptures and even adapting it to Hope to support Barack Obama – there was a secret meaning to this artwork.

Indiana’s original print uses three colours. The word Love, with the O falling on its side, is inscribed in fiery red capitals against planes of blue and green. Why blue and green? The answer is obvious when you know that Indiana was the lover of the abstract painter Ellsworth Kelly. Blue and green – this precise blue, this exact green – were Kelly’s most recognisable colours, beautifully combined in many of his most uplifting works. His 1963 painting Blue Green Red also has the third colour of Indiana’s print.

Love was created as Indiana’s relationship with Kelly ended. So it is a sad love poem, perhaps even an angry one. Yet it instantly became a beacon of idealism, optimism, youth and revolt. In 1967, the Beatles performed All You Need Is Love in a live international TV broadcast and Jefferson Airplane asked: “Don’t you need somebody to love?” It was as if Indiana reached into the heart of the age and plucked out its perfect artistic encapsulation.

It is not a dead word, by any means. “Love is power” declaimed Bishop Michael Curry in an address at the royal wedding on Saturday that invoked the spirit of the 1960s as a loving radical force. Is he right? The undying fame of Indiana’s artwork is proof that love still has blazing energy as both political and private forces, the milk of human kindness. We don’t need to ask why Love is such a beloved work of art, any more than why Stand By Me is still being performed to crowned heads in a gothic chapel.

Indiana was fascinated by the artistic properties of words. The glib description of pop artist doesn’t quite do him credit. He was part of a generation whose subtle experiments pointed not just to pop but also to conceptualism. Just as Jasper Johns painted flags and numbers, Indiana selected and isolated words such as Eat and Die. Theand there is stark word-images he created in the early 1960s are rooted in the Americana of neon signs and giant hoardings. Indiana also recognised that he was working in an older tradition of American art. In his 1963 painting The Figure Five, he pays homage to Charles Demuth’s 1928 painting I Saw the Figure 5 in Gold.

Andy Warhol said pop artists made art about “all the great modern things”, and there is nothing more modern, or American, than streets full of words and numbers. Indiana was a serious artist of the word, whose paintings, prints and sculpture powerfully home in on the totemic quality of language. Yet his most effective work goes beyond concept into the realm of ecstasy.

Love is a kind of spell. It is an enchanting word that spreads happiness and hope. There may be more revered, more respectable modern artists. But how many have hit the heart like Robert Indiana?