Can I change the button color?
Yes.
You can specify the color of the button by adding a bit more code when you add the Callback Request embed code on your website. You should add the following:
<script>
let CR = {
color: "#FF0000",
};
</script>
<script defer src="https://callbackrequests.com/embed/button.js?0.1.3"></script>
The color is a HEX value. You can pick a HEX color here.