Artists recognize that different painting mediums achieve different visual effects. However, if you take a good look at a photograph, or the scenery around you, you will notice that different aspects resemble different light and color sensations. It makes sense then that to achieve the variety we see in real pictures, we could use different mediums when we paint. I have found that if you get creative and avoid a purist standpoint, it is possible to create an impression using watercolor paints, oil paint, and acrylic paint in the same painting.
Decide where you want to use water color paints and apply those first. They work great for soft background light, still water, and spaces of sky. Use the water color paints anywhere you want transparent, luminating, solid color. Look at your photograph and identify areas where you want to create this effect and then apply water color paints there in your painting. In the painting shown here, I used them for the background light in the cafes, the sky, and the dark/first layer of water in the base of the fountain and the base of the fountain itself. While water color paints work great for very light colors, use oil paint or acrylic paint for the whitest of the white. The watercolor technique of leaving white areas blank looks uneven when you are using the multiple mediums. Anywhere you want a very dark but flat space, use thick, dark water color. In this painting, it was used for the darkest solid parts of the fountain. In general, use water color paints where you might be doing a lot of painting over them or details on top of them.
Look at your photograph again. Identify any areas of texture, especially dark ones. These areas call for oil paint which will literally pop off of the page, unlike the flat water color paints. In this painting, I used black, red, and green oil paint for the leaves of the trees to give them a "rough" appearance with the sensation of movement. For small, very white areas, such as the street lights in this painting, you can use oil paint or acrylic paint. Any dark, glossy areas should also be painted with oil paint. You should not plan to cover areas of oil paint with lots of detail as with the areas of watercolor.
Look at your photograph and observe the details. For these details, you will want to mix water color paints with some acrylic paint or gouache to achieve varying levels of transparency. The more acrylic paint added, the more opaqueness you will have. If you want it to be more transparent or colorful, use more watercolor paint and water. You can use this mix over the watercolor or the oil paint (after they have dried), but you will need a higher ratio of acrylic paint over the oil paint and do not expect to get smooth lines over rough oil. Paint layers of greater opaqueness over more transparent layers. Use light colored acrylic paint, especially white, mixed with water and colored water colors. A high level of transparency is good for water spray or reflections as illustrated in the spouts of this fountain painting. For completely white areas you can use acrylic instead of oil paint if you wish for faster drying. Lower transparency mixes are good for large, light-colored, detailed areas (such as the large house in this painting).