Coloured substrates are, of course, just as printable as yellowish or cold white paper, if the right grammage and surface quality is observed. Due to the fact that the colours are opaque ¹, i.e. slightly transparent, the colour of the paper also plays a role in risography, similar to offset printing. So there is a “fifth”² or, in duplex risography, a “fourth” colour, assuming that the two basic colours can be combined in two-colour printing to form a third colour. Adding the paper white, that is four colours.