The key to painting a successful monster face is the base color you select. It wants to be something that looks unhealthy, not something vibrant. Such as a dingy grayish green, a dirty yellow-green.
If you're going to be painting up a lot of monster faces, consider premixing the color so you can simply reach for the relevant container and paint. It'll save you time.
If it's only going to be one or two faces, blend the colors as you apply the paint. Start, for instance, with a 'normal' green which then you put touches of black onto, then blend gently. Don't worry about uniformity of color; monsters aren't into skincare and in fact adding some spots will add character to the design. If it's the first time you're doing it, practice beforehand.
Once you've got the main color working, make the eyebrows and lips quite dark. Add shadows under the eyes to add to the general unhealthy appearance. Emphasize the downward curve of the corners of the mouth, and add some shadow around the bottom under the nose.
