Drag the variable from the variable list and let go of it on top of the part you wish to color. Drop in either the part list or drop in the graphics window.

Select your parts of interest in the part list. Right-click and choose Color by. The list will allow you to color by a variable, or to pop up the color selector to select a single color, or to quickly color by Black, White, or Grey. Random will color each of the selected parts by a color chosen randomly from a relatively unsaturated color palette. Make transparent will give the part(s) a moderate level of transparency, and Adjust transparency will allow you to choose the transparency.