You can update the color of the Apply button by editing the CSS of the template Checkout style under the Manage → Templates → System templates.
In the CSS section, you can add the variable .button.promo-code-apply, and add the CSS parameters and it will look like this:
#payment-form-component a.button,
#term-selector-component a.button,
.receipt a.button,
.access-button a.button,
.button.promo-code-apply {
background-color: #000000 !important;
color: #ffffff;
}
The above code will make the Apply button black.
The Checkout style template is dedicated to the CSS for the checkout and the design of the Purchase button itself can be updated there.