Full Idea
Galileo argued that there is no such thing as heat (and hence also as colour) in the external world, so there is no reason to conclude from colour-blindness that we cannot know the truth about the world.
Gist of Idea
Heat and colour don't exist, so cannot mislead about the external world
Source
report of Galileo Galilei (Il Saggiatore ('The Assayer') [1623]) by Richard Tuck - Hobbes Ch.1
Book Reference
Tuck,Richard: 'Hobbes: a very short introduction' [OUP 2002], p.21
A Reaction
This key idea, taken up by Gassendi, Descartes and Locke, seems to me to be one of the most important (and, in retrospect, rather obvious) facts ever worked out by the human mind. Why does anyone still doubt it?