changing button text

Hello,
No it’s not hard coded. You can use loco translate to edit each string in the plugin. I know you’re not looking to translate but you can still use it to rename a string in your site language. Here is a quick quide https://localise.biz/wordpress/plugin/beginners.

Thank you

How to change the color of the button?

Hello,
It should be possible with a css code below:

.wpc-customize-product
{
background: #a23c3c;
}

with #a23c3c the new color code you want to apply.