Simple question on Navigator Raid Status (what do the colors indicate?)

[LakesideReader]

Ok, I must be an idiot because I haven't found this answer and I'm sure is blindingly obvious to the most casual observer.

What does the color on the Navigator 2 Raid status indicate?  Is it percent of space used?  Is there an error (no error indications on the drives themselves)?

I'm talking about the lower section of Navigator 2 window, when I select my TerastationII (TS-XEL/R5 Series) above, the status below indicates Drive (Raid 1) 1034.2 GM/1847.6 GM (56.0%) and it's YELLOW.

Below that is Drive 1439.3 GB/1847.6 GB (77.9%) and it's green.  This is a single drive I use for backups.

When selecting my older TeraStation (TS-TGL/R5 Series), the status below indicates Drive (Raid 5) is 577.4GB/696.5 GB (82.9%) and it's more orange than yellow.

If it's percentage used, then why is the single drive with 77.9% used green, not yellow?

Thanks for any help.  I'd post a picture but I'm clueless on how that is done.