Use the SAME paint color, for stairwell and upper hall. I have no idea how you plan to end it where the circle is drawn. Hard stop? You would need a few inches of "return" wall. Your stair does not seem wide enough to do that.

I agree with Jan. A transition there will never look right, run the same color up the stairs and down the hall.

The top of the stairs open directly into a sitting/loft area that I would like a different color. No hallway.

Well, same rules apply. A color transition will not work well. Depending on the layout maybe you can do one wall in another color as a feature wall.

Thank you for the ideas!

Do you have pics showing the left side of the door? I agree, the color should keep going past the circle you have, but perhaps we aren't seeing molding or casing that might be going there?

At least take the stairway color along the entire wall in question. You can paint the other walls in the space a different color perhaps.