From my window in Greenwich Village, on the morning of September 11 [2001], I was struck by the dramatic sight of a row of six or seven workmen standing on the roof of the building next door and look anxiously up at the sky. So beautiful was the vision of their golden faces in red helmets, dazzling against the blue sky, that I summoned a visiting friend to photograph the theatrical moment.

For one minute, before discovering that the cause of the commotion was the second plane crashing into the World Trade Centre, we relished the resonant colour chord of primaries, red/yellow/blue, and the dynamic angles made by the men's limbs. Arms were raised in shock and disbelief, heads held in hand and torsos twisted.

Like the depiction of combat between Greeks and Amazons on a classical frieze, the rhythmic pattern made by these exaggerated, primitve gestures, combined with a ravishing harmony of colours, held our attention. We responded to the decorative quality of the sight before knowing the horrific nature of the event.

Later at the studio where I paint, I reflected on the harmonious blending form and content in that image. This terrible scene, with its dancing repetition of red hats, contained the requisite elements for a painting, and I was seeking a way of organising my response to the tragedy through an ordered arranged of coloured shapes on a flat surface.

"We are suddenly faced with life-and-death issues," wrote Wade Newman in a letter to the New York Times shortly afterwards, "next to which the personal rant of so many artists from the past few decades appears self-centred, vapid and inconsequential. What we need are artists who can deepen our sense of our own humanity, who, through a balance of form and content, can create something as enduring as our will to survive."

I was reminded by this of the furore over Tom Stoppard's speech at the RA annual dinner last summer. Daring to suggest that a work of art comprising a thought or concept alone was inadequate, he bravely ventured: "The term artist isn't intelligible to me if it doesn't entail making."  The judges' choice of winner of the recent Turner prize is further proof that it is no longer necessary - or fashionable - to employ the use of a visual language to express an idea. 

Artists through the centuries have used a concise pictorial language to express emotion. In his treatise on painting of 1435, Leon Battista Alberti, like Matisse 500 years later, stated his belief in equipping the painter with a grammar.  Without this, the purpose of art - "the expression of man's soul" - could not be satisfied.  They believed that, with the correct arrangement of the elements of form, colour and composition, the soul of the beholder would be captivated and he could be elevated by his experience.

Alberti encouraged the painter to make studies of gestures and the emotions they portray, for only by externals, he believed, can we know the workings of the soul. By a conscious use of gesture, the painter will evoke the desired emotion in the spectator.

The paintings of the Italian Renaissance seemed to come to life on the streets of New York, and in the newspapers: Masaccio's depiction of Adam's gesture of horror and shame at his expulsion from the Garden, the beautifully articulated hands of the mourning figures surrounding Giotto's Crucifixion in Assisi or the aching tenderness of Fra Angelico's grieving Mary with her bowed head.

But the expressive power these works is conveyed not just through the minutest articulation of a hand or head, but also through the artist's understanding of the relationship between the elements. "Colours and lines are forces," said Matisse, "and the secret of creation lies in the play and balance of those forces...An avalanche of colour has no force. Colour attains its full expression only when it is organised, when it corresponds to the emotional intensity of the artist."

As an artist, my contribution to making sense of the mood of the aftermath was to take these insistent images of suffering, identify the underlying scaffolding of the form and turn them into some kind of harmony.

I noticed a recurrent theme in the triangular relationship between the head and the hands of a mourning figure. Whether the arms are raised in disbelief, the head bowed as if in prayer, or the face turned away protected by the hands, each establishes a geometric set of relationships, without which the colour would have no power.

My chosen sequence of colours must correspond to the mood or key of the painting. The red/yellow/blue trio of the workmen was like the blast of a trumpet: a vivid reflection of that harrowing moment of high drama. But another painting of a woman, head in hands, her elbows making a zigzag through the picture, demands a quieter, cooler key, to suggest the feeling of her being frozen in grief.

We painters have a language at our disposal for the expression of ideas and emotions. The laws of colour and light were the same for Alberti in 15th-century Italy - "Grace will be found, when one colour is greatly different from the others near it" - as they were for Matisse in 20th-century France - "it is only a matter of enhancing the differences, of revealing them".  The same logic applies in the art of making paintings today.

Newman's appeal in the New York Times for artists to make enduring images in response to the tragedy echoes the views held by Stoppard- and Matisse, for whom the most important aspect of painting was not the imitation of nature but "the transformation of perceptions into an enduring image".

There is plentiful subject matter in these displays of grief and compassion that can link us to the artists of the past. But we must also inherit or acquire the tools to express these emotions with the eloquence they deserve.