Shaped organic polymer hydrogels, especially contact lenses, are provided with opaque or translucent colouring so as to be capable of masking effectively any underlying colour, e.g. the natural colour of the eye. The hydrogel may be rendered opaque by precipitation in situ of a white or yellow water-insoluble, inorganic or organic material e.g. barium sulphate or an optical brightener and then coloured with a suitable dyestuff.