The 12 months comprise one section of the Tres Riches Heures manuscript. Painted between between 1412 and 1416 by the Limbourg brothers, it was completed after their death by Jean Colombe and the mysterious "Intermediate Illustrator." Several other works are attributed to them, but appear to have been lost. The three Limbourg brothers died of a sudden illness before the age of thirty in February 1416. Their brilliant colors were obtained from minerals, plants or chemicals and mixed with either arabic or tragacinth gum to bind them; vert de flambé, a green made from crushed flowers mixed with massicot, and azur d'outrême, an ultramarine blue made from crushed lapis-lazuli.
The Book of Hours prints come on 11 x 17 cotton rag paper in soft white with archival inks. Soft white is white but the paper has not been treated with bleaches or chemicals that would accelerate the paper's deterioration over time. Without comparison to other whites, it looks white. If you place it next to a brilliant white mass-market print, it will look slightly more cream.
You can find Book of Hours Tiles, a more extensive discussion of the individual months, a bestiary, unicorn tapestries, and other medieval art at William Morris Tile