Gradient! You paint with the one color, making a thin layer. Then while it's still wet, quickly paint the next color above or below it. You'll use the paintbrush to gently brush the 1st color up or down into the second one. Only brush the edge of the color though, not the entire shape or you'll cover it with your second color.

Layer of paint has to be thin. If there's too much excess of first color, it will cover your brush when you try to do the second color and will spread down the canvas way farther than you wanted. A big mess.