I've put together an easy eight-step guide to painting fish. I used cadmium orange (with some cadmium yellow added), ultramarine blue, mars black, and titanium white oil paints, but other choices of colors or paints can reach a similar effect. If in a time crunch or organizing for a class, a single fish could be easily completed within an hour or two.
1. Begin with "bean"-like shapes, or ovals that bend, for the bodies. Vary the sizes, to activate space and dimension to the scene. When you add circles for the eyes, remember you want them popping out, and facing you, the viewer.
2. Add fins. While creating flowing, organic shapes for the fins, consider the silhouettes of each fish, and how it interacts with the ones near it. Consider the effect of water, and how it moves and ripples through the fins. Have fun with the shapes, you can't really be wrong!
3. Mix black and blue together, add highlights to fins and bodies. There should be some blocks of shadow on the bodies, and lots of linear definition on the fins. Using the same colors we will use later in the background will help tie them together and give a sense of transparency to the fins. By leaving the original guidelines from the first two steps, it will becomes orange outlines, helping the fish "pop". Be careful not to over-work this step, it's easier to add darker colors later, because if you try to add light to dark, it can end up looking muddled and messy, especially if using oils.
4. Now we can return to orange by filling in the bodies and defining the fins with some sparse outlining. Be mindful of keeping your brushes clean (wipe clean after nearly every stroke), and apply the orange by working around the blue. Some blending as they touch is the goal, but too much mixing of orange and blue can create a nasty brown color. Note: the fins will be white with blue linear shadows and orange outlines, the bodies will be orange with blue shadows and white highlights.
5. Our final step to the fish will be our most delicate. We fill in the remaining spaces with white, while embracing the brush width so that the white is its own shape instead of an absence. Be decisive with each stroke; if you go over previous lines too much, it will mix instead of the intended subtle blend.
6. Now we outline the fish, finalizing the silhouette and giving them clearly defined shapes. I used black to make the fish "pop" significantly, but for a more subtle effect, use the black/blue mix from earlier. When outlining the white and orange, remember that dark colors overpower every light one it touches, so be careful not to blend like we did earlier (you'll just lose the light color entirely).
7. Apply blue swirls around the remaining negative space. I used black to outline the fish and blue/black for swirls, but if you used blue/black to outline, you might prefer just blue for the swirls or blue mixed with a tiny bit of white for an even lighter look. This is the first time you want to use a fairly big brush, to get nice broad strokes for the swirls.
8. Lastly, fill in the remaining space with the same color you used for the outlining. Lightly blend into your blue swirls by using a large, dry brush, and be sure not to apply too much paint. If you lose the swirls by overworking it, it's possible to pull back the color underneath by using a rag and your finger.