Having a theme or color palette in mind for your mixed media piece can help you decide what to use for your background. The reverse is also true- when you have a good background, it can help you decide what your theme will be. Here are some mixed media background ideas.
You can use corrugated cardboard to create an interesting background for your art. Tear the paper off on one side so that the grooved lines can be seen. My teenage son loves to save cardboard boxes so there is never a shortage of cardboard at our house.
Old “non-shiny” textbook or songbook pages make interesting backgrounds. You can find old books at library sales, garage sales and in second hand stores very inexpensively. The pages can be ripped, edged with ink or painted with craft paints. Another option is to stain the pages using brewed tea or coffee.
Using digital backgrounds. When I created, The Anointed One, a found object assemblage, I used a textured digital background image from Pixabay. I opened it it in my photo editing program and desaturated it, increased the contrast and lightened it. After I printed off some copies, I went over them with a wash of dark brown and black acrylic paint, letting the original design show through the layers of paint.
Natural fabrics such as burlap, linen or muslin can be used to cover your background. This works well for a rustic, natural kind of theme. You can either cover your whole background or just a few smaller pieces.
Using layers of transparent watercolor is another way to design an interesting watercolor background, you can layer transparent colors over one another, allowing the bottom colors to show through. A transparent watercolor effect can be achieved by mixing the color with water before applying it to your surface. Go here to check out more books on Mixed Media Techniques from Amazon.