Van Gogh close up

Saw The Olive Trees (1889) in the Scottish National Gallery: 

And was struck by the vibrancy of the colours he used. The contrasting interweaving he employs makes the canvas shimmer under the hot sun: 

And when did you last see a blue tree? However, if you look into a densely leafed tree on a sunny day, the light plays those tricks on you where colour be making its own rules.