The dynamic colour of three commercial reversible thermochromic inks were studied. Thermochromic printing inks change their colour according to a change in temperature. The two types of thermochromic inks are based on leuco dyes and liquid crystals. The thermochromic effect achieved by liquid crystals is quite different from the effect achieved with leuco dyes. Leuco dyes change from a coloured to a colourless state, or from one colour shade to another due to structural changes inside leuco dye molecules under assistance of developer. Liquid crystal inks change colour continuously throughout the spectrum, producing iridescent colours starting from red and shifting to blue part of the spectrum with temperature. The colour change results from the way light interacts with special arrangement of liquid crystal molecules to produce coloured reflection by interference, and with the variation of this structure with temperature. In this paper we will present differences between two types of thermochromic printing inks giving careful colorimetric characterization.