Stephen, both are awesome grills. Looking at condition, I would say the 18 has seen more sun. Knowing a little about color and sun light, my guess is the 18 has seen more sun. Yellow is a high burnout color to sun. If the pigmentation has yellow and green combination, yellow will fade and green will become dominant. Green is a non fading pigment.
I have had two yellows, both shined up different. The more weathered had less pop, even after an oxidation wax treatment. But it was still a Yellow and pretty!