New paint, much better results.
I went down to Kwal-Paint this morning with the little tin of paint I found last night. It still had the formula on it so the guy was able to just program his R2D2-like paint machine and 5 minutes later I had a 5-gallon bucket of the stuff. I masked and painted the walls on the stairs today, and now it's dry, the colour is identical in tint and sheen. Phew. Looks much better.