Does hair that is still attached to a person's head get cut if a Shardblade passes through it? If not, if that person had the Royal Locks could they change the color of the hair "below" the cut?
Yes, hair gets cut. It counts as dead in my mind--but not to someone who has the Royal Locks. They could only change below, as you state, and wouldn't get their hair chopped off. (I'm not 100% sure on this, but I Think I've mentioned in Stormlight before that you can cut things like shells on living animals with a Shardblade, but then it doesn't cut the flesh.)
With the Royal Locks, the individual would be able to change the color in the bit between the cut and the scalp? (In my original question I had meant "below" to mean the bit between the cut and the end whereas your answer uses it the opposite, I think?)
Distance between scalp and cut, yes you can change that. Otherwise every time you got a haircut, you wouldn't be able to change the hair. Maybe I don't get your question.
So just for clarity's sake is the following correct:
Let's say Siri's hair is two feet long. A shardblade passes through the hair exactly in the middle ("dividing" the hair into two one-foot sections). In your previous answer you said that while usually the shardblade would just sever the hair (leaving only a foot attached to the head) with the Royal locks the entire two feet would remain attached but only the foot on the "head" side of the cut would be able to change color.
Thank you, I just want to make sure there is zero doubt in what is actually going on.
I did misinterpret your original question. As a shardblade cut is likely to be wider than a piece of hair, I doubt you could cut the royal locks lengthwise.