Measurements with a tristimulus chromameter demonstrated that two 100-year old paintings still responded to light by bleaching and to the dark by yellowing. Hand-ground lead white paint samples were measured using a spectrophotometer to establish correlation between light/dark response and different oil processing methods and driers. Paint samples were prepared using freshly extracted linseed oil processed according to five recipes published between 1808 and 1845. Oil was treated with and without heating, and with and without driers (litharge (lead (II) oxide), sugar of lead (lead acetate trihydrate), and lead subacetate). A selection of historical artists' mediums - megilp, gumtion and copal oil varnish - were added to some of the hand- ground paint. Oil processing, driers and mediums were all found to influence the rate and degree of yellowing in the dark, as well as the final colour of lead white paint after light exposure.