I do not own Sherlock.
"You're not really human, are you?"
"I beg pardon?"
"You. You're not human, are you Sherlock?" John was so serious in his question that Sherlock actually looked up from his microscope (bee legs, of all things to be studying) and threw the most perplexed look he could muster.
"It's just…" John considered his words here a moment, knowing he had his flat-mate's full attention for a change. "It's like your eyes are mood rings. Have you ever noticed?"
"Yeah. Little stones, well, they used to be. I think it's chemicals now not real stones, but they change color on your body temperature, and it's supposed to judge your mood."
"Irrelevant, the human body does not change its internal temperature drastically enough for something outside the body, such as a stone set in a cold metal ring, to accurately indicate a change from joy to sorrow or such like." The tall man returned his gaze to his bee legs. John was losing his interest.
"I know that, but that's not what I'm getting at Sherlock."
"Then what are you getting at?"
"What color are your eyes? Because it seems that every time you look at me, they are different!"
Sherlock looked up again, genuinely surprised.
"They are violet."
"That's not a real color. People don't have purple eyes."
"Yes, they can. And I did not say 'purple' I said 'violet.' It's a term, not an actually color. You are a doctor John, I'm sure you've heard of it."
"No, I don't think I have. Surgery and heath didn't really play much into color theory."
Sherlock heaved a great sigh, turning to face his friend fully with great exasperation.
"If it really means that much to you. My eyes are a very light shade of blue, almost a grey, with a tint of violet. On most days, they are grey. Depending on what color I am wearing, the weather, and yes what mood I am in, they change colors. Mood is easy to guess. It is an unconscious habit humans possess. When they are in a certain range of emotions, their eyes react via how open or closed the lids are, as well as the dilation of their pupils. The more iris that is displayed to the light, the more color is reflected off them. Then there is the matter of what color is there to be reflected in the light itself. On days that I wear a shirt that reflects a green tint, my eyes are green. On days of blue, they are blue. Same with the weather. If the human eye picks up one color being reflected over another, that is the color they are for the day."
John let this information soak into his mind, going over his memories of what color eyes Sherlock had during what day and what was going on. He could see that working. Much like people with hazel eyes. If they wore blue or green, their eyes looked more green. Whereas darker colors, or brown, made them appear more brown. It's all a trick of the light.
"So your eyes are pretty much colorless."
"In a sense, yes. 'Violet' is the term for it, though I supposed 'grey' works just as well."
"Why was this a topic in the first place John?"
"I was starting to think that perhaps I had taken up a flat with an alien."
"There are no such things as aliens John."
"How would you know? You don't even know the sun is the center of our solar system."
"It's not something important."
"What if the Doctor came along and took you away as a companion and you HAD to know about it?"
And with such a simple statement, John had become the bee legs under the looking glass.