From the interview with Oliver Sacks about his last book Musicophilia: Tales of Music and the Brain in Saloon:

Hume wondered whether one can imagine a color that one has never encountered. One day in 1964, I constructed a sort of pharmacological mountain, and at its peak, I said, "I want to see indigo, now!" As if thrown by a paintbrush, a huge, trembling drop of purest indigo appeared on the wall — the color of heavens. For months after that, I kept looking for that color. It was like the lost chord.

This reminded me. I lived in Russia back then. I had a dream, and in this dream I saw an aquarelle with some building painted in a shade of yellow, a bit dirty yellow. The color was really important, full of meaning. There are lots of building painted in this kind of color in my city, and for long time I too was looking at them trying to find precisely the shade I saw in my dream. Of course, I never found it, and then it left my memory. For me, it wasn't like the lost chord, it was like a shade of a mood, a bit sad and nostalgic and homely.