Thin the transparent paint with an oil solvent. This step will make the paint easy to brush over the monochromatic opaque paint that has already dried. Also, the thickness of the transparent paint will determine the saturation of the color.
Brush the transparent paint over the entire monochromatic painting, or just over certain areas to enhance the color in those specific spots. Applying more transparent paint to one area of a monochromatic painting can make the opaque paint appear darker or lighter, depending on the color of the transparent paint.
Create layers of space by choosing either cool or warm colors to cover the opaque paint. Applying cool colors, such as blues, greens and violets, to a specific area of the painting will make that part recede. Applying warm colors, such as reds, oranges and yellows, to other areas will make those parts appear to move forward.
Apply several layers of transparent paint until you have achieved your desired effect. Note that applying more than one layer of paint may not be necessary. For best results, let each layer dry before applying the next one to see what the finished product will look like.