Primary colours: Ellsworth Kelly's unseen early drawings

Middlesbrough is quite a slow, grinding hike up the East Coast line at this miserably inclement time of year. The snow just never seems to stop flurrying against the carriage window. So I think of my visit to see this fairly small, and very unusual, show of works by an American modern master of abstraction as a kind of new year pilgrimage.

Ellsworth Kelly is one of the most significant figures in the development of post-war American abstract painting. When the words America and abstraction are uttered together, you immediately think of Abstract Expressionism, that muscular, groundbreaking style of painting on the grand scale with which we associate the names of Motherwell, Pollock and Rothko. Kelly, though he began painting when Abstract Expressionism was helping to define New York as the new capital of world art, is not exactly a member of that school; in fact, he is not really a pupil or teacher of any school.

Consider the works in this show, for example. These are all drawings from the 1950s – in gouache, graphite, ink and pencil – when Kelly had a New York studio. But they don't feel like the kind of works that would come out of a New York studio during the years when Abstract Expressionism was on its triumphal march. They are too fanciful, too quiet, too intimate, too sunny, too soft, too curvaceous, too engaged with the outside world, and too modest in size. Not a single one of them is more than one-foot square. They don't pressurise the onlooker, they don't pose, they don't posture. There is no wilfully aggressive brushwork, and in fact no sign of texture at all. They occupy a single, well-lit gallery on the first floor of the building, quite baggily too, as if to say: this is all there is, and it is quite enough. They are also very short on American machismo. Why do they feel so set apart when in fact he was in the midst of it all?

Because it was in 1954 – the year of the first of the 25 drawings in this show, which span almost a decade – that Kelly came back from eight years of living in Paris, where, having profited by the provisions of the GI Bill to study in Europe after the end of the Second World War, he had been learning to be an artist by looking at the likes of Matisse, Arp, Brancusi, Calder, Picasso and others, and travelling around and visiting the great cathedrals and momuments of France. (Kelly served in a camouflage unit during the Second World War which, you could say, was also quite a good preparation for a career as an artist.) So when you look at a drawing in this show called Study for Palisade, and wonder why this specific combination of colours, and the particular way in which they are working together, remind you of the clerical robes that Matisse designed and had made for the chapel of St Paul de Vence at the beginning of the 1950s, you would be spot-on. You will also notice that there seems to be some feel of European – perhaps even Mediterranean – light in these drawings. Yes, they definitely feel quite European, tonally. What is also interesting to note is how different these works are in mood from the younger abstract artists from the USA who were so recently on view at the Saatchi Gallery in London. In the world of the young, everything is coming at you all the time. Whap. Zing. Clonk. By comparison, Kelly seems to be moving at walking pace, as if plucking berries, one by one, from a bush. The choice is always his. He is not at the mercy of the world's teeming imagery.

In these works, Kelly is teaching himself about colour, form and line. What happens when you put that particular blue against this particular green? What happens when a circular shape is squeezed or cropped by a rectangle, or when a bulgy black circle gets its edges shaved off? How close should a shape come to one that you half-recognise from the natural world – or to a shape that you glimpse down a microscope? What kind of sensuous energy does that inject? There is an ongoing tension between the shapes and how they are framed – straight line butting up against curve like a bull snorting at a gate.

These works may be called drawings, but they are often exercises in patchings and matchings of colour. The colours chime and rhyme melodically, sensuously; they seem to swim into almost effortless conjunctions with each other as if they have been feeling their way towards each other for quite a long time. There is patterning, but not patterning with the regularity of Frank Stella's paintings of the 1960s. Kelly's regularities always prove to be slightly wayward, slightly awkward, slightly irregular. And that makes you smile. They have a fluid, pared-back shapeliness, a yielding sensuousness, about them. There is no spectacular trickery here, just attempts, time and again, to establish how a form establishes an identity for itself within a confined space. These works don't have the sombreness, the anxious, Freud-oppressed, self-regarding sobriety of works by those Abstract Expressionists. In short, they lack onanism and braggadocio. In fact, they point the way forward to that jolly trickster, Robert Rauschenberg. And as you stare at them, you come to understand why Kelly kept them by him, why he needed to have them with him. These drawings helped to define his future as a painter. They are small-scale maps for ambitious future journeys. Yes, they are almost ready to be scaled up to 10 times this size.

Why is this show of hitherto unseen drawings – yes, this is the very first time in 50 years that they have left his studio as a group, and gone on international exhibition – by an artist in his 86th year who lives these days in Spencertown, Upstate New York, here anyway? At the moment, there are no firm plans for them to go anywhere else after this showing – except straight back to the artist's studio. Which is where it came from in the first place. These are all works from Kelly's private collection, stuff that he has kept by him for the past half-century to learn lessons from; lessons in what to do next, and how.

And why are they being displayed here at Mima in Middlesbrough? Well, since it first opened its doors three years ago, Mima has been establishing quite a track record for itself as a place to go if you take drawing seriously. So this show of what you might describe as a series of small prototypes – in pen, charcoal, pencil, gouache and graphite – for a lifetime of paintings is a natural extension of what has gone before.

But Ellswothy Kelly won't have all of them back. Mima was recently given a grant of £1 million from the Art Fund to acquire works on paper, and Kelly agreed to sell two of the works that are currently on display in this show. So 25 arrived and 23 will be going back. Let's hope he doesn't miss them too much. Let's hope that his future development as a painter during his energetic ninth decade isn't put in jeopardy.

To 21 February, Mima, Middlesbrough (01642 726 720; Visitmima.com). Admission free