Piano hits the wrong note: London's new Central St Giles complex is striking but bereft of joie de vivre

In one mighty Technicolor bound into St Giles High Street, sandwiched between Covent Garden and Bloomsbury in central London, the architectural deity Renzo Piano has become the capital's latest blingmeister. The Central St Giles mixed-use scheme is a luridly jolly prequel to his forthcoming Shard tower at London Bridge. Can this shouty polychromatic architecture really be the work of the 72-year-old designer whose museums have become masterful demonstrations of fine-boned modernist decorum?

The great man says he didn't want Central St Giles to take "selfish possession of the site". Piano is referring to the fact that the development is pedestrian-porous at ground level. But when his building covers more than 400,000 square feet and is covered in a Pixar-perfect sheen of 134,000 green, orange, red and yellow glazed terracotta tiles, that is a bizarre claim to make.

Has Piano brought a high-rent hallucination to the northern edge of Covent Garden or has he achieved a coherent architectural intervention? Actually, neither. St Giles High Street, used by the Romans as the main approach to what is now the City of London, then in the Middle Ages as a last halting place for those to be hanged a mile west at Tyburn Tree, has been architecturally Crayola'd. The Angel Inn, where the condemned sipped water, and St Giles-in-the-Fields church are humbled; the only things that offer a sensual counterweight to the new development are the garish hoardings of the Shaftesbury Theatre, announcing its Burn the Floor dance show.

If Piano's buildings don't quite sear the street scene beyond recognition, they do imprint the vista with a bumptious, laser-etched precision. And this sets a planning precedent. Are we going to see a deluge of Pantone buildings by infinitely less talented, novelty-hungry architects and developers who wouldn't know a Bridget Riley painting from a Victor Vasarely colour chart? You know the sort: they default to jazzy façade patterns of coloured metal to bring a touch of vapid class to those parts of our towns and cities where legions of Tiny Tims are waiting to thank them by declaring: "God bless us, every one, for this architectural turkey."

Central St Giles highlights something else. Its arrival, which has been orchestrated carefully, obscures the fact that there are more impressive recent commercial buildings in London whose architecture has stayed under the radar simply because it is less dramatic. They are not show ponies and they set more interesting urban development precedents than Central St Giles. We'll come back to them in a moment.

It's not that Central St Giles isn't significant architecture. On a "future's so bright you have to wear shades" level, these facades are certainly vivacious, and we might even think of them as a kind of strident public art. After all, those tens of thousands of terracotta panels are part of what is described as an architectural "chassis". In other words, they have been appliquéd to the external glass and steel envelopes of the buildings – which makes them an architectural artifice. The trouble is, the glaze of the terracotta imparts a distinctly contradictory metallic aura and, most crucially, the overall effect is neither beautiful enough, nor surreal enough, to be truly remarkable. Despite Piano's care with the layered patterns of the facades, the effect is striking but not resonant. Nothing about these facades lingers in the mind. They are, oddly, bereft of joie de vivre.

But there is a bigger issue. This major conglomeration of buildings, articulated to look like loosely joined blocks, raises big questions about just how far fag-end fillets of the West End can be pumped up architecturally by developers. It is the way Central St Giles has been positioned so carefully that is so depressing. On the tiles: "The colours are a nod to the lively sights and sounds of nearby Soho." This is bunkum. The colours – apparently inspired by album covers seen in local shops – are hardly specific to Soho.

The laundered prose of the scheme's sponsor, Stanhope, is typical of the Orwellian newspeak that is involved in so many big commercial projects. Central St Giles, it murmurs, "brings heart and soul into a forgotten part of central London's urban fabric". Forgotten by whom? And in what way will the impending insertion of Zizzi, Fishworks, Sofra and the grilled Japanese skewers of Bincho into the ground-floor segments of Central St Giles bring heart and soul into the development?

The only thing about Piano's scheme that contributes to its urban context is the way it has established visual and physical links, via its central courtyard, through the site. This space, calmed by dove-grey facades, can be seen through the double-height, glazed ground-floor segments of the building. From St Giles High Street, for example, there is a view straight across the groundplane to Bucknall Street. Piano has also been civil in the way that in the apartment portions of these buildings there is no visible external difference between the top-rent and affordable accommodation.

But if you want a truer architectural beauty, and a frisson of surreality, you must repair to New Bond Street. Here, Eric Parry's brilliant reinvention of a whole block of linked buildings running between New Bond Street and St George Street has given London its most exquisitely fashioned façade, on what is unquestionably a benchmark redevelopment.

If you compare Parry's serpentine faience ceramic façade tiling with Piano's Central St Giles elevations, there is no contest. Parry's ceramics exude artistry and lusciously lucent gradations of colour. This is a complete fusion of architecture and art. Furthermore, though the façade introduces something quite new and faintly startling to New Bond Street it is absolutely at ease with the architecture around it.

Parry's combination of contextual response and meticulously tailored architectural innovation has made him the go-to architect for developers who want unique commercial buildings. His Aldermanbury Square building has the most finely detailed metal façade in the City – he compares it to a David Mellor knife. And the masonry load-bearing structure of his Finsbury Square office building possesses the most precisely cut stone seen in London since the 18th century.

And now, even Parry has serious competition. David Chipperfield's forthcoming office building in the massive King's Cross Central development will bring a compelling mixture of toughness and elegance to the scene: the raw edges of the building's concrete floorplates and beautifully figured cast-iron columns will stand clear of its glass facades – a fusion of the classical and the 19th-century industrial heritage of the site. And in the last two years a much younger architect, Patrick Lynch, has shown the imagination to join these established stars with his highly artful but contextually sensitive design schemes in Victoria Street. This kind of architecture doesn't need marketing hype or Las Vegas dye-jobs.

Renzo Piano's Central St Giles project has put commercial architecture on the media map for the first time in many years – not since Sir James Stirling's No 1 Poultry in the City have we encountered such a wilfully vivid mixed-use building. Yet there is a risk that Central St Giles will convey a false sense of worth by suggesting that the design of so-called rent slabs is all about dramatic, "because you're worth it" architectural implants.

Architects, developers and planners will serve our towns and cities better if they face up to the fact that commercial architecture need not be predicated on glib non-ideas about the hearts and souls of forgotten places. They must instead address what Eric Parry describes so elegantly as "the finesse of the relationship between the mercantile world and very brave architecture". That is the real challenge. And gift-wrapping buildings isn't the answer.

For further reading:

'Piano: Renzo Piano Building Workshop 1966 to Today' by Philip Jodidio (Taschen)