Hello!
I hope you are having a nice day~
Also I hope I'm not asking a thing which was answered.
I'm rereading Kuroshitsuji and I noticed a really odd thing. It made me confused, so I want to ask you about it - In chapter 108 O!Ciel has a nightmare. Then he wakes up, but he hasn't got a mark of contract on his eye! Did Yana write on Twitter that it's a drawing mistake? I personally don't believe it's a drawing mistake, becouse Sebastian seems really shocked. Is it foreshadowing?
Dear Anon,
Don't worry, I was not asked this question before, but thanks for being considerate! It's always much appreciated.
Now about O!Ciel's eye in chapter 108 when he just woke up from the nightmare. Also don't worry, there's nothing wrong with it; that's just how shocked eyes are drawn in manga.
We can be sure that it was not a drawing mistake, because it is very unlikely Yana would make this mistake twice.
In drawing shocked eyes, the contrast between the iris and the pupil is exaggerated to express the constriction of the pupil. Often the iris is not coloured either for this very purpose.
See here for example. Sieglinde's eyes are green and are coloured grey in the manga. When she's shocked however, we see the pupils constricted and the colour gone. It is not like her eyes are suddenly not green anymore.
Another demonstration in Sebas and O!Ciel again, where both have their eyes drawn in white, without it meaning that their colours are gone.
There's not really a rule of when to draw what as what. It's just a thing people who draw a lot do: "Whatever seems adequate to communicate what needs to be communicated at the moment."