https://upload.wikimedia.org/wikipedia/en/9/90/Captain_Blood...
I think it was a demo because it only had an animated intro I didn't recognize from other games, a menu, and the game was a plataformer with a large sized alien or android that could walk and shot. But it restarted (or crashed?) after a few seconds.
https://www.youtube.com/watch?v=UOl_fBWIuac
The video seems to match your description quite well!
So, Metal Mutant (1990-1991) came before Flashback and Legend of Kyrandia (both 1992). I can't recall a previous game that has such impressive artwork. It seems they modified the gradient from blue-orange (sky-desert reflections) to colors that match the environments, which makes it kind of monochromatic. So the prototype looked even better as I recall.
I like how ChatGPT can be used to to do fuzzy "queries" like this and come up with the right answer:
https://chatgpt.com/share/4cfcffdc-48e3-4780-a55a-091836c79c...
It guesses wrong to begin with, but then I give it the additional information that the game is French, and it gives me the right answer.