This way, your edges stay soft and blend smoothly. Push with the lighter color until you get a smooth blend. If the dark color gets too light, add more dark again, feathering to keep from getting harsh edges.
Any lighter color will push a darker color out if the dark color is soaked enough. You can always keep layering colors as needed until the blend looks right. While blending, always keep a layer of scratch paper under your work in case the ink soaks through.