Colour is the real winner in a tricky contest

A good artwork and a good poster are not necessarily the same thing. A poster has to be arresting, to grab your attention away from the visual cacophony of urban life, but art should aim to create a sustained reaction, prompt a deeper engagement.

The commissioned artists thus faced a tricky proposition - and it is fascinating seeing how they have responded.

Much of the anticipation has focused on Tracey Emin, whose poster finds her at her most sensitive, reflective and optimistic. Her poster is a pretty vignette but it lacks the striking power a poster needs. Chris Ofili combines a similar whimsy with much more graphic clout in his mythical beast-cum-woman framed by a classical urn.

But for me, the four best posters are among the most colourful. Bridget Riley's horizontal lines rotate her dazzling vertical stripe paintings of the Seventies and reminded me of the bands of colour that accompany the tense excitement of a photo finish.

Rachel Whiteread's coloured circles juggle the Olympic rings but also draw on the marks left by cups and glasses on tables - she gives an everyday human warmth to the subject.
Michael Craig-Martin's is perhaps the most striking - vivid colour with a stark, exhilarating message. But for sheer visual pleasure and clarity, Howard Hodgkin's Swimming, a tumble-turn captured in a few swathes of sumptuous blue, wins the day.