The gas space heater in the center bottom is the cause of the yellowed discoloration on this wall section.
It already looks better.
I used mostly ColorPlace Exterior Semi-gloss latex white with a little FolkArt Outdoor opaque acrylic Thicket. I didn't try to get a uniform mix because when I sponge, I like different shades mixed in. I'm not finished, but it looks a lot better to me. In addition, the next time I think it looks dirty, I'll be able to wipe it down. Yea Exterior!
If I didn't do anything else to it, it would still look better than it did. But I plan to let it dry for a while and then sponge some soft color in to suggest flowers.