Below is a shot of my current shadow-box in process. As you can see it is much larger than any other one I have tried which presents a number of challenges but also offers interesting possibilities.
What you see here is the front of the box covered with the underpainting. I under-paint most of my work and my favoured color for the underpainting is Burnt Sienna. Burnt Sienna is a relatively new color (first used in painting in the 19th century) and because it is an iron oxide based pigment it offers excellent coverage and remarkable stability. It is also an excellent underpainting color because it is dark enough to serve as a linear base where necessary but not so dark that it deadens the overcoats or makes them hard to paint.
-A note on underpainting - I first became interested in an undertone for painting when I was in my teens and discovered the work of the 19th century English watercolour painter David Cox. Cox was an excellent painter (though not as fine as his contemporary Thomas Girtin) who often painted on rag paper or packing paper which had a brown tone. A number of paper makers have produced papers which are homages to Cox and in the 70s and early 80s there was a very interesting rag paper that was a fairly dark and absorbent that I used a number of times. I don't know who produced it but the production run ended and so I looked for substitutes and began to soak my watercolour paper in coffee to get an undertone. Because coffee essentially dies the paper it offers little interference with the watercolour that you lay on top of it. I eventually realized that underpainting has always been a common technique which many of my favorite painters employ. Even Bonnard (an often neglected painter) used undertone. Anyway, in the early 90s when I began to paint with acrylics (and sometimes oil) I experimented with various undertones until I began to favor burnt sienna. Underpainting provides depth and solidity to a painting that is difficult to achieve without some undertone.
Below you see a detail from the front of the above shadow-box. It is a sequence of photos which offers a brief insight into the process.
You can just see the drawing on top of the burnt sienna. The upper rectangle is painted in Naples Yellow which is one of my favorite colours (easily made with Cadmium Yellow, Cadmium Orange, and White) The rays of the sun are straight cadmium yellow.
Cadmiums are great colours but they offer poor coverage. Thus to reach a solid colour with a cadmium such as the rays of the sun above one is required to make up to five coats of paint. The black here is Mars Black which is highly recommended when painting over an undertone. Mars black is another iron oxide colour and therefore offers better coverage than Ivory Black. Though you cannot see it in these photos, I often coat my blacks (which I don't use very often) with light coat of Ultramarine Blue. This technique creates greater depth and a slightly translucent effect like the black feathers of a Magpie.
You can only see a bit of detail here but I have begun to add a little more depth to the colours with a cadmium orange at the tips of the rays and a lightened mix at the bases. (The addition of white is often a favorable advantage for the creation of coverage over an undertone)
Below the face is formed and I am putting the decorations in. Above the face I laid down a streak of cadmium red (made in several coats) and then painted the words (written backwards) with cadmium yellow.
Below the words are finished with small streaks of lightened ultramarine blue in the joints of the letters and a green (created with a number of colours) in between the letters, leaving just a small amount of the red visible.
And here I have gone through the same process in the yellow segment and connected the two sections.
The detail is now, more or less, complete and will later be integrated into the other sections of the painting.
I hope this has been interesting to someone, and if you have any questions feel free to ask.