textured surface before glazing
underglaze layered surfaces before glazing
underglaze layered surface after glaze firing
Generally I fire each piece at least three times: first I bisque fire newly formed work; next I add a layer of underglaze, sometimes multiple colors or colors that highlight certain textures or details; finally, after the first layer of underglaze has been fired, I add a second layer of underglaze that is wiped away from the raised surfaces.
This wash-away process can be used with stains or glazes as well and doesn't necessarily require the extra middle step of applying underglaze to the entire piece. Any highly textured surface can be highlighted in this way by adding color (glaze, underglaze, stain, even ink or paint). Color can be applied evenly, then washed away from the raised surfaces causing low areas to fill with applied color and raised areas to reveal the clay body underneath. I find this wash-away method to be the most natural way to highlight complex surfaces. It is also a relatively quick, if somewhat messy approach.
I also highlight or alter surfaces by careful and precise application of color onto the surface. I sometimes paint a highlight of underglaze onto a raised surface of an applied sprig. In past work I more frequently painted underglaze on smooth surfaces as an applied decoration. In the past I have also mixed color (oxides or mason stains) directly into the clay itself so that textures can be created with colored clays. This technique works best with a light colored clay like the porcelain pictured below.
The "seeds" in this porcelain "pod" were each created using several differently colored porcelain clays. The seeds were then built into the pod. A clear glaze was applied and the form was fired with no liquid color applied at any stage.