The underpaintings above are oil paint on canvas (from a canvas pad), collage
and NeoColor II water soluble oil pastel on canvas, and collage
on acrylic wash (from left-over paint) on watercolor paper.
Since I'm an abstract painter, my underpaintings are just about getting color down, collage and mark making like the one below. Generally, I use the colors of an underpainting as a guide for the colors I paint or glaze over top, however, traditional underpaintings typically use one color; grey, sienna, red iron oxide, sepia, yellow ocher, and others.
NeoColor II water soluble oil pastel on watercolor paper.
Salt was added while the pastel was wet to give the spotted effect.
There are some great resources available for free online about techniques for underpainting, such as this blog post from Jerry's Artarama HERE and a YouTube video from The World of Art HERE. A few of my favorite examples of underpainting in books are the write up about artist Juan de la Cruz Machicado in Painter's Wild Workshop by Lynn Leon Loscutoff and the "From Drawing to Painting" chapter of Bill Creevy's The Oil Painting Book. Don't worry that the references speak about oil painting, the same techniques can be used for acrylic painting and other mediums, too.
Happy Arting! Tristina :D