You can modify default colors of your WordPress theme using color picker. You can change them in bulk like invert all colors, use grey scale etc.
Theme Tweaker is a free WordPress plugin. If you like modifying colors then install and activate it. Visit Appearance -> Theme Tweaker and start tweaking your theme colors.
You can preview the modified colors and save stylesheets locally. It can generate the child theme for the active WordPress theme and using it you can leave your original WordPress theme untouched so that you can run updates without any tension.It does a lot more and also offers a pro version.