Theme color scheme
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.
Note: color names and samples are available at the Color table page.