http://www.vam.ac.uk/content/articles/c/constables-oil-sketches/ Looking at these oil sketches on the computer –I cannot tell if there is a colour ground nor what that colour may be.
Landscape with a double rainbow John Constable (1776-1837)28 July 1812
ref: http://www.vam.ac.uk/content/articles/c/constables-oil-sketches/ this appears to be mainly a blue green in colour with overlying light greens and pale whites or yellows.
Brighton Beach, with colliers, John Constable
ref: http://www.vam.ac.uk/content/articles/c/constables-oil-sketches/ this appears to have a blue grey ground, even onto the beach.
and : Barges on the Stour, with Dedham Church in the distance, John Constable
ref: http://www.vam.ac.uk/content/articles/c/constables-oil-sketches/ appears to be painted with a purple blue ground with areas of white canvas left for breaks in the clouds and beach edge.
Rubens –I found a few oil sketches on Bridgeman education
The Meeting of Abraham and Melchisedek, c.1625-26 (oil on panel)
Artist: Rubens, Peter Paul (1577 – 1640) ref: http://www.bridgemaneducation.com/ImageView.aspx?result=3&balid=936986
Here there appears to be a pale blue wash underlying the images of the figures and so with the image:
The Four Evangelists, c. 1625-26 (oil on panel)
Artist: Rubens, Peter Paul (1577 – 1640) ref: http://www.bridgemaneducation.com/ImageView.aspx?result=4&balid=937116
playing with creativity 