purple
grimwerk: So far, I have:
Exactly one purple.
Exactly one yellow.
Exactly one of the following colors occurs twice: green, aqua, red
At most one blue.
We also know it's impossible to have black or white.
Guess 2 shows that of the 6 colours:
red blue green aqua purple yellow
5 are correct and 1 is wrong.
Guess 1 shows that if black was correct, then 2 of (blue green aqua purple) are wrong. This contradicts Guess 2, therefore black is wrong.
Guess 3 shows that if white was correct, then 2 of (green aqua red purple yellow) are wrong. This contradicts Guess 2, therefore white is wrong.
Hope the above is clear (and I didn't make a mistake anywhere).
So there is no use guessing black or white now.