Issues with changing the colour of buy it now/add to cart button

Go to your Online Store → Customize → Settings → Custom Css
and paste this code in the custom css section

button#AddToCart {
background: red !important;
color: white !important;

}

.shopify-payment-button__button:not(.shopify-payment-button__button–branded) {
background: #0002ff !important;

color: white !important;

}