Here is my example with the steps to create these.
1. Divide your 9 x 12 white paper into 6-7 sections with pencil, then paint them different colors. I used the Alpha Biggie Tempera Blocks, but any paint would work.
2. The second class we outlined the colored areas with black marker and created simple designs in each with the black marker. I told students to think about what they would see under the ocean to come up with some of their patterns.
3. They used oil pastels to accent the black patterns and add even more color and detail.
4. The last class we drew the fish on black construction paper and outlined them with Crayola sticks (wood-less colored pencils) You could use oil pastels for this as well. We kept the shape of the fish and details simple as there is a lot going on in the backgrounds. The black is used to contrast all the colors!
Here is the student's work.