In this way we can use color tints (other than grays) in our custom theme (e.g $primary: $indigo-600; ), w/o having to extend $theme-colors and redefine $theme-colors-rgblater. This could be done in the project variables file, but i believe most developers just include a full local copy of scss/_variables.scss before the the original default file and change the values the need (maybe also removing the !default flag) so that it's easier to track changes and custom values during upgrades.