I'm color blind. I think that's way I don't get the game. I first though there are just the colors dark green and yellow, but than after not being able to pick up the key I realized: it must be light green instead of yellow (at least that's what I'm guessing)
The two screenshots look the same for me, so I'm guessing one of them is yellow?
To make your game color-blind friendly use colors with a clear contrast (like yellow-blue or Yellow-Black). Additionally, it might help if the key has an animation.