For monoprint, I create a pattern with the brush and heavy body acrylics, painted directly onto the plexiglass/trashbag surface. Then, I press the paper into the paint and pull a print. This process is very good for showing brush texture and interesting patterns. Typically I paint the paper a shade first (in this case the paper is yellow with fluid acrylic), let it dry, and then monoprint a complimentary color in heavy body OR fluid acrylic (in this case ochre in heavy body).
You can also monoprint first on white paper, let that dry, and paint over it with watered down fluid acrylic so that the monoprint shows through. Experimentation is the name of the game, using colors that sit next to each other on the color wheel, or analogous colors, look pleasant together because they are closely related. Orange, yellow-orange, and yellow are an example of analogous colors. Sunflowers by Vincent Van Gogh is an example of a painting that utilizes this type of color palette.