Here is a technique where you can create interesting effects with a single brush and multiple colors.
Using a flat brush, first wet your brush with clear water. Then pick up the main body of color, working it into the whole brush as you pick up the color. Then lay the side of the brush down into a different color well. Picking up color only one side of the brush.
In all three samples shown, I used yellow as the main body color. The first sample shown I picked up orange on the side. I then cleaned my brush and created the second and third samples where I chose to pick up a dark green.
Swash your brush across watercolor paper and watch the two colors from your brush mix with each other.
You can create beautiful flower petals. Multi-colored leaves for almost any plant, quickly and easily by side loading your brush. Give it a try and have fun painting!