1. Go to Online Store->Theme->Edit code
2. Asset->theme.scss.css->paste bellow code in bottom of file
@Perennial I think, unless I'm mistaken, that this will set all prices to red (except for compare at prices) i.e. when the product is not on sale the price will also be red.
Thought that might be the case. Unfortunately it's not that straightforward, but the up side is that @KarlOffenberger has already provided the answer in another post. You need to scroll down a few posts to get to it as the original part is about something else.