Many thanks for your reply and playtesting.
I thought about describing the goals of the game and concluded that this would simplify the game. But since the game is already hardcore enough, I think it may even be beneficial. Also I could not come up with an organic and narrative introduction with a restriction of three sentences. Probably I really should write the goals and the basic description on the game page.
P.S. Rainbow color is just a hint that there are seven colored keys =)
UPD. Fixed most recommendations.