Image for a missing variant

I don't think this is expected behaviour, but I can't find any similar examples to check
In the product catalog on my development site
js-ca.net/potluckcms
there are 2 colour (beige yellow) and 6 style characteristics for the breakfast cup in the west coast series (see attachment).
But the Eagle and Two Salmon styles ONLY come in beige, not yellow.

So when I click on the yellow/eagle variant combination I see the image for the product parent (also see attachment) .

How would I change this behaviour to show no image or a message that the variant is not available
I could create an image to use for non-available variants and use that, but I wonder if there is an easier way?

Many thanks

Jim

EDIT:
Further to this question. I just realised that there is no place to enter, in the variants pane, an image for a missing variant. So how do I show an image that says "There are no Eagle items in Yellow" when someone clicks on the Yellow and Eagle radio buttons?

Thanks, Nicolas.

At the moment the site is just a Product Catalogue running in Catalog mode, with no option to buy.

You earlier gave me a fix to show the "no stock" message in catalog mode . But I lost that code change when I changed templates and the message no longer shows. I'll put it back when we finalise the template that we want to use.

I might just create 24 "dummy" products for the missing variants. It's not a clean way of handling the problem, but certainly less confusing than what I have there at the moment.

"No Stock" is not exactly the same as "product does not exist" so it would be good to have a different way of handling non-existing variants in a future release! )

Thanks again

Jim