This should go without saying but it’s surprising how sometimes it isn’t the case: the clearest action on a product page should be to buy that product (or add it to a cart/basket). If you've got the user this far and interested, they shouldn't have to guess at how to give you their money.
The action should be a button and it should stand out, ideally using a colour that is used sparingly elsewhere on the site. Don't blend it in, don't make the wording unclear, don't prioritise other actions on the page (like adding to a wish list). This page has one primary goal so help the user to reach it.
Avoid having your button greyed out, even if you want users to do something like selecting a size first, as they’re likely to think they can’t buy it. If they click the button without selecting a required size, you can just pop-up the colour picker with a message. If something actually is out of stock, you could replace it with a link to a contact form so they can ask to be informed when it is available again.