Hi there!

I have some suggestions for you! Reason can be one of them:

You may try to go in Advanced > SCSS > Compile SCSS to CSS to activate it. Once you're done with your colors you can remove it.

So, you might try toggling the custom style setting on/off during change to see if there isn't just an old file that's loading and isn't yet rewritten by the compile function.

You may try enabling compress the CSS and Javascript to get it to work.

Sometimes the main reason for not showing the changes is Adblock software installed on the computer. After disabling the Adblock software the changes are shown immediately.

Clear Joomla and Browser cache(Most important)

Hope this helps!

Best Regards