Building colors and depth is what the creation of marble patterns is all about. Dark to light is the preferred order of color application. This allows the deepest tone to set the mood for the marble finish. About half as much middle tone as dark is used and only about ten percent as much light tone. Veining in marble finishes can be done with any number of tools.
Most traditionalists use feathers to create veins, but script brushes do a wonderful job as well.
For fun, try using really bizarre tools like cotton swabs, skewers or toothpicks to lay on thin broken lines that resemble the cracks characteristic of natural marble. Strong textural elements are often desired in paintings involving natural subject matter. One easy way to create a crackle look is to use a reductive method of surface decoration. The organic, natural breaks of the surface will provide visual interest and implied texture. For stronger crackle patterns, allow the topical material to stay in place for a longer time.