If you're looking to use a consistent color, whether in a button or text throughout the site, and you're looking to match the exact shade or hue, you can copy-paste the HEX code. Every color has its own unique HEX code and is visible when selecting a color.
For example, here's how to use a button color HEX code that's copy-pasted from a different button on the site...
Hover over the existing post where the button color you want to use is displayed, and click on the "edit" option.
Hover over and click on "BUTTON COLOR" to reveal the HEX code inside the color editor.
Highlight and copy the HEX code inside the box next to the "#" symbol.
Locate the post where the button color you want to change is displayed, and paste the HEX code inside the box next to the "#" symbol. Click on "Post" to save the change.