You shouldn't really paint the whole tree, including all the branches and twiggy growth, but you can use a watered down emulsion (water based paint, and not a vinyl emulsion either) on the trunk. This is sometimes done on Birch to increase the whiteness of their trunks for decorative effect.

its also done near me on oak trees that are close to the road on the trunk in quite a wide band .

Confused slightly. How will painting help those senses normally aimed at in such a garden? However, instead of painting a tree, what about PLANTING a tree with a prettily coloured stem such as prunus serrula or betula utilis var. jacquemontii? Worthy

and ofcourse we have more than one sense to i mite add worthy .