Red was fierce. It was passionate. Cas still didn't understand why it was used to symbolize stop on a streetlight when it looked like it had so much energy. Maybe it was because when you were covered in red yourself, chances were you stopped.

Orange was fall to Castiel. It held none of the connotations it did for humans. None of the tea and pumpkins that Sam had told him about. But it was still fall. The leaves had always changed, always died slowly on the branch before breaking off and spiraling to the ground for their first taste of dirt and grass and mud. Castiel wondered how many angels had fallen in this way, their faith slowly dying while they tried to cling to the life they knew before crashing to the ground tasting the dirt and the grass and the mud. And yet also the sun.

Yellow meant warm. Meant the sun. Meant brothers' joined grace intertwining with his and raising up, up, up. It meant a hawks beak, sharp and dangerous. It meant a pair of shoes Castiel had seen a lady wear once, tall and slim and so, so bright. Dean had snorted at them, he remembered with a smile.

Green was freshness. Was age. Was life. Was death. Was a sprig of rosemary sprouting in the cracks of a tall stone building he had visited on a whim. In a manner not unlike spinning a globe and letting his finger land where ever it may. Except the finger was him, going round the planet fast, so fast he could almost forget. So fast that he could focus on the burning of his joints and the sharp singing of his grace. His grace. Green was also the colour of Dean's eyes.

Blue was the colour of his own eyes. And his vessels eyes. The line between the two was getting more and more blurred. The body wasn't just something to slip on anymore. It wasn't just a slightly ill-fitting suit. Instead it was his. Sometimes it felt a little tight, but all he had to do was get up and move a bit. The cramped feeling would leave soon after. He supposed that would fade with time. And all he had to do was stand up and walk around a little and he was fine again. Blue was also the ocean. And sometimes sadness, though he wasn't precisely sure why. Some blues just felt sad.

Humans had a 'brain-twister' where they challenged each other to come up with a new colour, one no one had seen before.

Cas saw colours like those every day, his favorite being a colour with no name in the human language. It was a colour that came before white. It perched on the very tops of mountains and the very tips of cat's whiskers. Sometimes you could catch a glimpse of it swimming on the underbelly of a fish, or flashing by on the end of a deer.
It was by far Castiel's favorite, and he was loathe to leave it behind.
As hard as imagining a new colour would be, Castiel thought forgetting them would be much more difficult.