One of my favorite ways to apply gestural line and color is with the use of washes.  Washes are essentially colorant - commercial stains or oxides - mixed with water (sometimes a little frit if it's a refractory).  Here's where the science gets fuzzy.  I don't usually measure the colorant out other than by teaspoons (or parts thereof) into water by cups or portions of cups.  I'm generally looking for the color of the water.  True, this makes it hard to duplicate.  But, the nature of the wash is variable.  As you apply a wash, picking up more water than stain, they will become progressively stronger if you aren't careful. 

The key to making them work is having run tests on the washes (at least record amounts - teaspoons and cups - when you begin) on a variety of surfaces and clays.  It's amazing what a simple wash will do to alter a glaze, slip or clay surface.