understrap/sass/theme/_theme_variables.scss

// Use this file to overwrite the basic Bootstrap variables and add your own variables
// To overwrite a Bootstrap variable you don´t have to touch the Bootstrap folder. 
// Just copy a variable from /sass/bootstrap/_variables.scss, paste it here and edit the value.

//
// Color system
//

$white:  #fff !default;
$gray-100: #f8f9fa !default;
$gray-200: #e9ecef !default;
$gray-300: #dee2e6 !default;
$gray-400: #ced4da !default;
$gray-500: #adb5bd !default;
$gray-600: #868e96 !default;
$gray-700: #495057 !default;
$gray-800: #343a40 !default;
$gray-900: #212529 !default;
$black:  #000 !default;

$grays: (
  100: $gray-100,
  200: $gray-200,
  300: $gray-300,
  400: $gray-400,
  500: $gray-500,
  600: $gray-600,
  700: $gray-700,
  800: $gray-800,
  900: $gray-900
) !default;

$blue:    #007bff !default;
$indigo:  #6610f2 !default;
$purple:  #6f42c1 !default;
$pink:    #e83e8c !default;
$red:     #dc3545 !default;
$orange:  #fd7e14 !default;
$yellow:  #ffc107 !default;
$green:   #28a745 !default;
$teal:    #20c997 !default;
$cyan:    #17a2b8 !default;

$colors: (
  blue: $blue,
  indigo: $indigo,
  purple: $purple,
  pink: $pink,
  red: $red,
  orange: $orange,
  yellow: $yellow,
  green: $green,
  teal: $teal,
  cyan: $cyan,
  white: $white,
  gray: $gray-600,
  gray-dark: $gray-800
) !default;

$theme-colors: (
  primary: $green,
  secondary: $gray-600,
  success: $green,
  info: $cyan,
  warning: $yellow,
  danger: $red,
  light: $gray-100,
  dark: $gray-800
) !default;