TM Drive Icon was yellow - is now blue - ?

Hi, I just wanted to begin using my old drive for my new Mac.
Yesterday I deleted the one file from the drive (old TM backup)
by moving it to the trash. The following "Empty trash" lasted very long...

Then a long backup followed. About 12 hours for 240GB - is this normal?
I noticed that the drive icon which always was yellow
,
was now displayed in blue! What did this mean?

Today I opened the drive and the color of the icon is yellow again.
Can anyone please explain this?

Thanks!