The theme main colors can be changed from app/sass/_vars.scss

In this file, you will be able to see the primary color variables for many site elements.

$brand-primary: #999 !default; 
$body-bg: #fff !default;
$text-color: #535353 !default;
$link-color: $text-color !default;
$link-hover-color: #ccff99 !default;
$btn-default-bg: #66cc66 !default;

Also, you can integrate these variables like in the following example:

h6 {color: $headings-color;}