Hard disk default icons
Long story short: the white drive represents a "removable disk", and the orange drive represents an "external disk".

Think of a removable disk like a thumbdrive, something small that people would usually throw in their pockets or carry around on a regular basis, or in your case, an external SSD.

An external disk is usually the exact opposite of the removable disk: it's usually something that sits on your table that rarely ever gets moved like a hard drive bay or a RAID rack, or in your case, an archive HDD.

I'm not too sure how the system determines which one to associate with which file, though I am fairly certain the icon is chosen based on the drive's capacity. If you'd like to see where these icons come from (or their names which give clues on what they are associated with), you can find them in /System/Library/Extensions/IOStorageFamily.kext/Contents/Resources, or for your sidebar icons they can be found under /System/Library/CoreServices/CoreTypes.bundle/Contents/Resources along with many other system icons.