The First Six Books of the Elements of Euclid In which Coloured Diagrams and Symbols are Used Instead of Letters for the Greater Ease of Learners

"Byrne ... considered that it might be easier to learn geometry if colors were substituted for the letters usually used to designate the angles and lines of geometric figures. Instead of referring to, say, 'angle ABC,' Byrne's text substituted a blue or yellow or red section equivalent to similarly colored sections in the theorem's main diagram."--Friedman.