
This is fully adjustable from .GNUstepDefaults using the key "gameover-seconds-to-bw-fadeout". Set this to any float value you want and this is how long it will take for the game over screen to turn to black and white. The game over screen lasts for 30 seconds, so that is effectively the max you can set it to. Higher values will work, but the full transition to b/w will not be complete before the game resets to the title screen.
You don't like the effect? No worries, just set the above key to 0.0 and all is back to how you knew it.