Toning a Canvas
This technique is called 'toning' a canvas, and can be a variety of different 'start values'. Toning a canvas can create an expressive effect when bits of the base color show through the final layers of paint. It's up to you, but its fun to experiment with different types of tonal values and see what suits you or your composition.
Toning is done by thinning a base pigment with a medium to make it watery and provide a very thin layer of color to a painting surface prior to painting the full composition. This follows the concept of 'fat over lean' - meaning its better to start with lean, thinned values of pigment and build layers on top of that with pure pigment or thicker oil+medium mixes.
What is the difference between gesso and a toned surface?
Gesso is a acrylic-based 'primer' to provide a barrier between the raw surface (linen, canvas, wood, paper) and oil-based pigments and mediums. Whereas a tonal layer goes on top of a base gesso layer to tone down the white reflectivity of the gesso layer.
What is oil-based ground, and what makes it different from gesso?
Tonal layers can also be created with a mix of an oil-based ground medium and a small amount of pigment. Oil-based ground goes on top of an acrylic gesso layer, and can provide a better bonding layer with oil-based pigments.
Common pigments used in toning a surface
Yellow Ochre - This is the most common that I've seen used in plein air painting. Light and vibrant areas that shine through upper layers mimic bright, sunny light reflectivity.
Burnt Sienna - Warm earthen pigment used in land-dominant plein air paintings. Brings a believable, complexity to color layers because it mimics earthen values.
Raw Sienna - More of a darker earthen pigment that falls in the middle of the color wheel. Commonly used in studio paintings to block in darkest values or large shapes in a painting.
Quinacridone Red - I've seen this used very, very thinned out, sometimes with a touch of white mixed in to tone the canvas hot pink. Can provide a dramatic expressive effect.