Down Over Up, Fruitmarket Gallery, EdinburghJoan Mitchell, Inverleith House, Edinburgh

A mischievous conceptualist's work is still laced with wit, but pales against a thrilling colourist

Martin Creed has got a cheek, hasn't he?

The mischievous conceptualist is infamous for The Lights Going On and Off (Work No 227), 2000, which demonstrated that the Yorkshire-born artist has swagger and gall if nothing else – a kind of "I did this and you didn't", as he perseveres with his gloriously absurd vision. Then came Work No 850 (2008), his astonishing and eloquent art-in-motion piece, for which sprinters zoomed through Tate Britain's Duveen Galleries.

So is his new and recent work, Down Over Up, on show at the Fruitmarket Gallery as part of the Edinburgh Art Festival, just as audacious? And, crucially, is his art still relevant?

Creed's work is still laced with a degree of warmth and wit, making it as accessible as ever, from the choir of voices that holler in the gallery lift (Work No 409, 2005) to the staircase transformed into a synthesiser (Work No 1061, 2010), each step sounding a different note underfoot. Laugh in the lift, have a giggle on the singing stairway ... if nothing else at the Fringe grabs your fancy.

These pieces reflect Creed's core concepts: the desire to make his art as democratic as possible, with sound works created in situ on the stairs by the visitor; the idea of growth and order denoted via plotted, incremental stages; and, significantly, the realisation that with progression comes limitations. Every move forward eventually hits the buffers: there are, after all, physical and conceptual restrictions to everything.

This rigorous ideology is stretched to its limits via stacks of chairs (smallest item always perched on top), a tapering tower made of Lego, a series of astutely executed paintings of tiered ledges, and a military line-up of 13 quirky cacti that increase in size.

Running up and down these scales should make us ponder the whys and hows of repetitive actions, giving our minds and bodies a complete workout so that engagement is total. But the opposite happens here. The art of tedium is, for the most part, well, tedious. It's not gratifying, just truly tiresome.

At rare moments, though, the mundane can still become marvellous in his hands. There's something elemental, even comforting, about a row of nails jutting from a wall (Work No 701, 2007), a piece which brings to mind Sol LeWitt's minimalist wall drawings. And plywood has never been more majestic, crafted into a finely layered shed-sized solid that devours the space (Work No 841, 2007).

What once seemed sincere and subversive about Creed's approach now seems rather contrived. Is his work still pertinent? Not on this showing. This art-world jester may well get the last laugh but the joke is wearing very thin.

An exceptional show of seven paintings and five works on paper by Joan Mitchell at the Royal Botanic Garden makes Creed look like an art-world minnow. It's staggering that this Chicago-born abstract expressionist, who died in 1992, is only now getting her first show in a British public gallery.

And there could not be a better setting for Mitchell's deftly handled deluge of form and colour than Inverleith House, with its crisp neoclassical interiors and verdant lawns. Aspects of nature triggered emotion in Mitchell, a lyric painter inspired by poets such as the US writer Frank O'Hara; her abstract art not only breathes in this dazzling space flanked by natural features, it pulsates and erupts.

Straddling the hard-edged US expressionist sensibility of the 1940s and an elegiac European outlook (she left Manhattan for Paris in 1959 with only one work on show before that year), her work is at once contained and expansive, as muscular and sinewy as anything produced by Jackson Pollock and Willem de Kooning, her brushwork as sophisticated as that of Cy Twombly and Vincent van Gogh.

Feel the energy behind the knots of paint that protrude from the canvas but be aware that every staccato stroke, each seemingly spontaneous trickle of pigment, has a purpose. Her work is nothing if not nuanced, unencumbered but controlled.

The interplay of tones and hues rules in this art ecosystem. Mitchell is an incredible colourist, as demonstrated in the oil Untitled (1958), a canvas where brown and crimson slabs dovetail with turquoise daubs. This meshing of colours is finely tuned in the pastels, creating a sense of extreme depth and perspective in Tilleul (1977).

The most affecting piece is First Cypress (1964), a piece dominated by a hovering mass encrusted with gouges of dark green impasto which splinters into rippling calligraphic lines across the canvas expanse. Tracking the changing terrain of the painting, from the denser passages to the broader, deliberate, outlying strokes, is a demanding, exhilarating experience. In fact, it's hard to walk away from every work on show here, such is the clout and complexity of the compositions. The exhibition, the stand-out show in Edinburgh this year, confirms Mitchell as a pivotal post-war practitioner.

'Down Over Up' (0131 225 2383), to 31 Oct; Joan Mitchell (0131 552 7171), to 3 Oct

Next Week:

Gareth Harris squares up to the monumental stone sculptures of Atsuo Okamoto