It sounds really strange, but I can't describe the timbre of the piano. Violins can be wispy, or scratchy, or flowing. The saxophone is mellow and breathy. Double basses have intense vibrations and fill the room. The drums are nearly toneless and can rattle or boom. But the piano just IS. Maybe it's only me but I can only compare the sound of a piano to fresh, clean water: how would you explain the taste of water? I can't. 

Something also really weird that I found is that when I hear someone play a piano, it's like hearing someone talking with my own voice; the voice in my mind. It might be that I play the piano so much but it really hit me the other day as I heard one of my siblings play: they're talking with my voice, the piano! I was ecstatic that I could--and had-- connect with a box of wood filled with strings and hammers.

“The piano keys are black and white but they sound like a million colors in your mind.” – Maria Cristina Mena. And, I would add, they sound like the voice in my mind.