The Realms of Light and Shadow are not directly above the part of Shyish where the Harrowmark is, they are on almost the same horizontal plane, so the "sun" always appears low in the sky. So Hysh is always filtered through a lot of dusty air and appears to be like what we see at sunset.
Each morning Hysh and Ulgu rotate around each-other so Hysh is in front and the land is bathed in horizontal orange-red light for a while. In the evening Hysh is slowly eclipsed and less and less light will fall on the Harrowmark until night falls.
So in the Harrowmark it will only ever be night or evening, and never full daylight.
I imagine the movements of the Mortal Realms relative to each other to be unfathomably complex, so that it would take generations of study to begin to be able to plot them. If it's even possible at all.