Do any colour coding conventions exist for system architecture block diagrams? If not, who/what would be considered the de-facto standard? Is block diagram the correct terminology in this context?
Specific hardware/roles of interest are caching/persistence tiers, load balancers and public/web servers.
Some (conflicting) examples:
WikiMedia system architecture
A MapReduce system design pattern
While you might find someone who has created a conventions including color, you will not find it respected universally in IT. Block diagram design techniques are often shared but specific and consistent conventions will be unique to IT departments and individuals.
Of course, with specific types of diagrams such as circuit diagrams, you will encounter more rigid standards. This does not apply in IT.
There are conventions within types of diagrams but not to the level of detail you are requesting. If you want to explore types of diagrams, this Wikipedia article is a good start.
The IEC 61131 standard includes a specification for a Functional Block Diagram in the context of programming languages. Nevertheless, as detailed before, this will not be universal.
If you want examples of how other people diagram their infrastructure in IT, the Rate My Network Diagram Web site is a good start.
