An analysis of pigments with the same pigment colour identification number on the colour index reveals a startling fact. Samples of Burnt Sienna and Red Iron Oxide, both of which contain Pigment Red 101, are placed next to each other on a board and drawn down to reveal the range from the masstones to the undertones of each colour and they look very different. Burnt Sienna is a darker, warm red with a yellow undertone and Red Iron Oxide is a colder red, yet they are made from Red Iron Oxide pigment. What accounts for this difference? The variance is a result of pigment particle size and distribution.