I use the wet into wet watercolor technique to blend one color into another color.
1. I start off with a 1/4 sheet (11 x 15") piece of watercolor paper taped on all four sides to my board. I use my favorite 1 1/2" wash brush which is a Winsor Newton brush, a blend of sable and synthetic bristles. A 1 inch roll of masking tape (or two 1/2" rolls stacked as shown here).
2. I prepare my palette with an ample amount of rich color puddles. Maimeri Blu's Permanent Violet Bluish, Ultramarine Deep and Primary Red Magenta.
3. With a clean brush and clean, clear water, I wet the paper flat from top to bottom. Evenly distributing the water. Then I slide the roll of masking tape under the top edge of my board, so my board is now at an angle.
4. While the paper is still wet I add color. I start with a blend of the violet and ultramarine. In long horizontal strokes from one edge of the page to the other I add the color. Do not stop in the middle. Do not start in the middle. It is important to keep the strokes long and the full width of the page.
5. While the color and paper is still wet, one must work quickly. Do not allow your paint or your paper to dry. I switch up my colors adding in a bit of ultramarine and overlap my colors and I continue to keep my strokes long. Adding in some of the magenta.
6. I continue on down to the bottom of the page and have shifted the color to a pale magenta. I leave the board at an angle and allow the paint to dry completely. With the board angled, the colors will slowly blend and fade into one another as they dry back.
If you keep your brush, paper and paint consistently moist with the same amount of water you will be able to blend one color into the next without creating harsh lines or back runs.
Give it a try ... and good luck!