Well simply put... if we translated it very vaugly...
*bella* donna = *night* shade
Donna
mirra = shade
yellow?
Mirra *bella* = yellow *night* ?
Well a plum is dark in color... and maybe the color just before night?? I doubt if tolkien dove this deep into thinking about all of this... but maybe bella could be translated as "night" or "plum"...
... maybe im completely wrong... but if the chain is meant to be translated literally... i guess that would be the closest thing to it...