What is RTFM, ofc?
"Read the fucking manual, of course". DarzaR is a known jerk, just ignore him.
I'm talking about Jagged Alliance 1, as in screenshot.
That's the thing, you *don't* leave the inventory screen to the main game screen because then you get the question mark. You do all these steps in the inventory screen.
1. Pick the camo kit so cursor changes to a hand carrying the kit.
2. Click on the body of the merc (the place pointed by the arrow in the screenshot).
3. The vest changes to camo colour.
Put the camo kit down, you can now exit the screen.