Many, but not all themes have a Theme color scheme. For aesthetic reasons, LEGO designers often choose to unify the sets and impart family resemblance by using certain colors.
For example Blacktron has a color scheme of black with yellow trim, trans yellow canopies, and trans red accents as can be seen in the example set Renegade Runner.
This is most common in space themes but is often seen in castle and aquazone as well.
Fan created themes often, but not always, have theme color schemes as well. For example Pre-Classic Space is white with blue trim and transparent canopies.
Note: color names and samples are available at the Color table page.