I almost always do my underpaintings in burnt sienna and lead white. The warm earthy orange of burnt sienna works well with the colors I tend to use. They are both low-oil content pigments, which is important for the underlayer. This follows the ‘fat-over-lean’ rule, which states that higher-oil content pigments must be used only in the top layers of a painting, with the leaner layers beneath. If you don’t follow this rule, the painting won’t dry properly, and some layers might crack.
If I want to achieve certain bright colors, however, I often use glazing. A glaze is a layer of paint, mixed with a medium, which makes it transparent. (Some pigments are naturally transparent, which enhances the effect.) When painted over another color, the glaze allows the under-layer to shine through. In this case, I wanted a bright green for the glass bowl, parts of the vase, and the fluorite crystal. Glazing viridian green over cadmium yellow produces a vibrant green. If I were simply to mix cadmium, yellow, viridian green, and lead white, the resulting mix would look dull by comparison. Because a glaze is transparent, it will always look clearer and more luminous than opaque body color.
Everything in the underpainting is lighter in value than the finished image will be. I think that the top layers of color look brighter over a lighter base. You’ll also notice that I don’t worry too much about detail in this stage of painting. All of this will be covered up, so there is no point in spending a lot of time on things that will be re-painted. I’m also careful to stay in the lines and not lose the drawing. I’ll need those lines as a guide later on. One last reason to do an underpainting is just to get some paint down on the canvas, with no pressure for it to be correct and finished. An oil painting like this is layered and evolves over time. It’s important for me not to feel pressured to be brilliant, especially at the beginning. There is always time for that later!
I’m finished. Now I need to let it dry for at least a week before I can paint over it.