You could argue it's supposed to always have rings of yellow fur, and it can THEN also light those up, but I dislike this interpretation. The rings glowing is more impressive and impactful if they normally AREN'T there by contrast. Plus think about it, having bright yellow fur rings by default is clearly shit for nocturnal camouflage. The rings should be black like the rest of the fur when not glowing.

I really think Umbreon shouldn't have visible rings most of the time. Especially not in 3d games. It should dynamically glow and unglow, glowing bright during attack animations but only faintly flickering most of the time and complete turn off when fainted or when about to sneak up on someone.