Search 'painting restoration potato' you'll find a group of people who have been fooled by the idea of cleaning a painting with a freshly cut potato and some have used onions. While the initial affect is the desired one, the painting will darken substantially over time. The way it works is the carbohydrates in the potato will react with the linseed oil in the paint (linseed is the binder in most oil paints) practically removing the oils from the paint. The reason varnishes and paints change colour and darken is because they oxidize, sort of like rust forming on steel. Removing the varnish and even the oils in paint is the same as removing the clear coat on a car, you are inviting damage to the paint. Varnish and oil protect the pigments (essentially crushed minerals, dirt, or even bone) from oxidizing and destroying themselves. The potato trick might make the painting look nice for now but soon enough you wont have a painting left to save.
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