External Hard disk colours on the Desktop

I have 3 HDD on my desktop. 1 = Mac HD, 2 = external HD for my Time Machine, and 3 = external HD for CCC.
On switching on both external HD's are a sort of yellow but after a while the Time Machine external drive becomes an aqua colour, however the external HD for CCC stays yellow even after doing a "clone".
This doesn't seem to affect any functions but simply out of curiosity why does one external HD change colour and the other not? Does anyone know?
Both external HDs are connected by Firewire 800 and are "daisy chained".

It's in the OS. The TM drive is identified as a TM drive and changes to the aqua color. Because the CCC drive looks to the OS just like any other external drive with files on it, that drive is NOT recognized as a backup and therefore doesn't change. If you turn TM off, the drive will revert to a normal yellow color because it's no longer identified with TM.

You didn't ask "Why" but I'll venture a guess...I suspect it was Apple's way of trying to minimize the accidental ejection of a TM drive while TM is running. One more "alert" that this drive is different from the others.

Thanks MacinWin, that sound logical - Garoolgan

On my machine the externals are clearly orange. I'm wondering if you might have your contrast or brightness set incorrectly (or if I have)

Chas_m, the icon only changes if TM is turned ON. It takes a while for the icon to change, but it becomes blue with three stick figure images on it. I'm not sure what triggers the change, but it does change. I've seen it (I don't have TM turned on now, I use TimeMachineEditor to run it on a schedule I prefer).

CORRECTION: I think the drive icon is blue with the TM symbol on it, not three stick figures.

Here is the icon for a TM Drive. Taken from Yosemite 10.10.4 today after I turned on TM and waited a bit (longer than 30 minutes, I was out of the house for a while).

The TM drive generally uses the icon you referenced. Most external drives have the icon that Chas_m referenced. The blue drive with stick figures is the icon normally used for network drives.

@Jake do you have the Time Machine backups on a Time Capsule or other network device? In that case I think those drives mount as blue network drives (see above) and turns the normal TM green when the Time Machine sparsebundle is opened to start a backup.

No, the TM drive is FireWire. Because I leave TM off and let TME schedule it I get the normal yellow drive. To get the image, I turned on TM and waited. I had to leave, but I think the icon changed when the first TM backup kicked in. When I turned TM off, the icon stayed blue, but I had to unmount the drive and when I mounted it again, it went back to yellow. I think I got the idea it had figures from my wife's Mac which is backed up to a network drive.