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.
I'm in the same boat. I tend to use adobe color [1] to pick colors. You can give it a color, and it will give you complementary or contrasting colors. It is great for making background gradients or finding a color that "goes" with background for those of us that can't really tell.