My favourite Pokemon is Umbreon. Who had the best design was also Umbreon. It just looks BOSS, cool and cute all at the same time. It's Shiny Form is just very good, as the moon would have that kind of colour. As for Yellow, they may represent an eclipse - A moon blocking the sun. Umbreon's design can prove that it represents the moon and/or any kind of Eclipse. We have some information about Umbreon's design from Bulbapedia - "Umbreon stalks prey silently at night. As it awaits its victims' approach, the yellow rings on its body begin to glow. During the day, they blink on and off. They are often seen on roofs and in trees staring at the moon.". The Yellow Ring part is maybe to do with an Eclipse. But yeah, better not go off too far into the Umbreon subject, or else this post would be a little inadequate. So yeah, that's just my thoughts.