The 4th Dimension – Wittgenstein on Colour and Imagination

What does it mean when we say we can see
colours? How do we use them and how do we learn their
names? It is not at all clear whether we all experience the
same colour when looking at the same red rose.
Nevertheless we use the same word and we do
understand it when someone else does. We know that red
and yellow make orange and that blue and yellow make
green, but can we imagine a reddishgreen or a
yellowishblue? And why haven"t we got a concept of a
white transparent glass?