The general aim of this paper is to elucidate some aspects of Newton's theory of light and colours, specially as presented in his first optical paper of 1672. This study analyzes Newton's main experiments intended to show that light is a mixture of rays with different refrangibilities. Although this theory is nowadays accepted and taught without discussion it is not as simple as it seems and many questions may arise in a critical study. Newton's theory of light and colour can be used as an example of the great care that must be taken when History of Science is applied to science teaching. An inadequate use of History of Science in education may convey to the students a wrong conception of scientific method and a mythical idea of science.