When you for example install this theme, not only the dracula theme is added to admin/customize/themes but also a dracula color scheme is added to admin/customize/colors
You don’t have to guess. If you don’t want to install the theme, you can check whether the palette is offered as part of the base palettes you can choose from, when you create a new color scheme.
You can also find the colors in the theme’s about.json. For example
Or you can use the browser’s inspect tool to find the hex codes of the colors of the color scheme you selected at a forum. Just right-click on the page, choose “Inspect,” and you’ll see the CSS styles, including the color values, which are usually almost at the bottom.