Hi @pipoulito,
When using WooCommerce it wouldn’t be possible by default to grey out a variation. The only option by default would be if a variation is Out of Stock it would still display the variation but that variation would not be selectable to be added to the customer’s cart.

Image Link: https://cloudup.com/ccLPBD3hCFQ
The only other option would be to completely hide a variation if it was out of stock. That can be achieved by enabling this setting below from WooCommerce > Settings > Products > Inventory.

Image Link: https://cloudup.com/cmphjTivG7i
You’re welcome!
Glad to hear that @stuartduff’s reply was helpful here!
If you have any further questions, I recommend creating a new thread.
How did it not occur to the folks who wrote woocommerce that things come in colors and sizes, may not be available in all combinations – and making the customer guess, scroll, see ‘out of stock’ scroll back, guess again until they find the combination that is available was a dumb thing to do?
Thanks so much for the feedback,
Your opinion is helpful in case anyone else interested in this topic is looking for a solution.
I recommend creating a new thread in case there are any other questions or comments since this thread is tagged as resolved.
Thanks.