Hey,
You could use the wix-stores-backend API, specifically the getProductOptionsAvailability() function. You can use this function to determine the availability of a product.
You would then just have to add two text boxes to the page, one for each message in the colours you want and set them to be hidden on load. Once you do that you can add code to set the correct message to show() depending on the product availability.
Hope this helps!
Dara | Corvid Team