Ozone is a pale blue gas at standard temperature and pressure. It forms a dark blue liquid below −112 °C and a dark blue solid below −193 °C. Satellite instruments monitor the ozone layer, and we use their data to create the images that depict the amount of ozone. The blue and purple colors are where there is the least ozone, and the greens, yellows, and reds are where there is more ozone. (wikipedia entry)