I have the same issue. Not being a designer I can spend hours tweaking CSS color values, only to come back the next day and think it looks awful again. Two things have helped me: 1) using
https://coolors.co/app and just trusting it, 2) creating SASS functions that derive secondary colors from a primary one. I currently don't have it at hand, but it boils down to making a series of lighter and darker versions of a given color, where, very importantly, the lighter ones are also less saturated than the shadows.