Owen
The answer is that the purple doesn't dye the wood, but I always have a sense that there is a faint tinge of colour perceptible. On the basis of a comment in an article by David Charlesworth I now look through the bottles on the DIY shop shelves to pick the palest ones - believe it or not, every now and then you get one with less colour in.
In the US you can buy anhydrous ethyl alcohol (colourless) for this exact purpose, from Ron Hock for example. Unfortunately I am not aware of any UK stockists of the Hock product.
In the UK you can get industrial methylated spirits which is colourless - my father worked as an industrial chemist before he retired and used to bring small quantities home from work for me. With that source of supply now dried up I have been on the lookout for somewhere to buy small quantities of industrial meths for a couple of years, but to no avail.
If someone does know of a UK supplier I would be extremely interested to hear.
Ed