It's a trend across Microsoft Office, Google Drive, and LibreOffice: the iconography for the word processor is primarily blue, and the spreadsheet app green. Any specific reason for this, or just a random industry trend?
I believe that this is just following the idea that the market leader (in this case: Microsoft) had.
There is an interesting article about the history of MS Office software here: http://www.thewindowsclub.com/history-evolution-microsoft-office-software which may explain more.
Originally, teal colour was used in the icons of both MS Word and MS Excel applications. The icons had a different shape, though. At some point (as the article screenshots show: in 1999) the developer decided to use different colours to easier tell one from another.
At that time, due to the obvious market leadership, the applications themselves were equated with their purpose (so instead of saying: "Send me that rich text document!" we used "Send me that Word (file)!" and similarly "spreadsheet" = "Excel (file)" etc.).
What was behind the choice of a specific colour may be hard to identify now, but the sole fact of making them different allowed to make easy connection between the application icon and the file icon, as both of them contained the same elements (namely: the file icon contained the icon of the app opening it). This reduced the cognitive load used to identify the right subset of files in a long list of them (look for green if you want a spreadsheet and blue if you want a text document).
From now on the convention was followed about using green for Excel, dark blue for Word, and other dedicated colours for the rest of the applications in the package. The competitive solutions started following many conventions regarding the UX of the software, compatibility, formats etc. Between them, the colour convention was apparently followed as well.
One more thing regarding the choice of colours is that for the User who, in many cases, was familiar with Excel greens and Word blues, leaving the same convention would, again, limit the cognitive load when moving from one solution to another.