Hey Dan,
This might not be the best way, but if in a pinch, you have everything set at w3-amber and you want it set at blue, you can use ctr+H to find and replace multiple ambers to blues, or just add the css page to your site, vs connecting to it, so you can modify the css yourself. then find the class you used, and change its attribute.
example
.w3-amber{color:#000 !important;background-color:#ffc107 !important}
to
.w3-amber{color:#000 !important;background-color:blue !important}
but that would set you up for confusion later, but if you needed a quick fix, use it.
I prefer the find and replace method, as doing the above will change everything on your website that was suppose to be amber to blue, and you might of wanted some of it to stay amber.
If you find yourself wanting to change theme colors often, set up your own theme settings, so you can change the theme colors easy and in one spot.
example.
theme-primary { code here }
theme-secondary { code here }
theme-offset-color { code here }
theme-textColor {}
theme-visitedTextColor {}
theme-backgroundColor {}
and so on.
then just change your colors in your css (just the theme settings) each time you want to change your sites color themes.