The technique you use to apply the paint when creating a color-field painting is, obviously, a matter of personal preference. Using a brush gives you ultimate control, but pouring can produce glorious streams across a canvas.
Using a big canvas is common in color-field painting because it increases the visual impact. This necessitates covering a lot of canvas before the paint dries if you want to avoid hard edges where you don't want them. Ensure you have sufficient paint to hand before you start to avoid having to stop.
Don't use too small a brush. You don't want to be painting little stripes of color back and forth, back and forth, fiddling to get it all blended (as in the bottom photo).
Using a softer brush, such as a varnishing brush, rather than a stiff bristle brush helps get smooth, texture-less brushwork. Strive for a balance between blended color and visible brushwork. Too much texture will distract from the beauty of the color, but a touch (such as along the edges of an area of color) adds visual interest.
See also: Tips on Painting on a Big Canvas
