When working with poured in place countertops, you can marbleize the surface using Color Hardeners.
Pre-Colored countertop materials (SCP’s Duratop System) are generally Polymer modified for added cohesive and tensile strength. This is a great system for countertops, but it also adds a finishing dilemma in the way that the Polymer makes the surface sticky to the trowel.
The trick is to let the material set with an open (mag trowelled) surface for about one hour for a standard one and one half inch thick top (Temp. dependent), this allows the Polymer to tighten up and lose some of its stickiness.
After the cure time has been allowed, then mist surface with water, and slick finish with steel trowel. At this point you have a multitude of finishing options, you could stamp a texture into the surface, work in bits of another material, etc.
A very nice technique is to sprinkle Color Hardener over the surface in an irregular pattern; I will usually use a small spoon to apply the sprinkle in what resembles marble veins. Then mist over the Color Hardener with water, completely wetting out material.
Then, pass over the surface with a steel finishing trowel again, the trick is to pass in one direction, and to not overwork as that will bleed the colors together more leaving less delineation of the veins. Then, after a ten day cure time, you can also Acid Stain the surface if desired, or use Stone Shades immediately.
— Marshall Hoskins
Specialty Concrete Products, Inc