This particular string follows a specific set of guidelines outlined in the JSGF specification. For a more user-friendly explanation, you can also refer to MDN's resources.
To simplify, we can dissect it as follows:
#JSGF V1.0; serves as a header, indicating which version of the specification to adhere to. It usually remains constant.grammar colors; denotes the (custom) name of your grammar.public <color> = establishes a rule named color that is accessible publicly. This means that your grammar can be imported by others, and this particular rule can be utilized by them. In this context, color represents the rule name, necessary for referencing from another rule (as seen in the example later).aqua | azure | ... represent the options that align with this rule. The | symbol signifies "or." Therefore, upon recognizing any of aqua, azure, ..., it will match the <color> rule.
A more intricate example involving referencing is demonstrated as follows:
#JSGF V1.0; grammar greeting; <greet> = hello | welcome; <name> = Alice | Bob; public <statement> = <greet> <name>;
However, in terms of practicality, I experimented with MDN's Speech Recognition Demo and noticed that browsers might not fully utilize the grammar yet. Upon inspecting its source code, it became apparent that the recognition.onnomatch function is never invoked, rendering the grammar somewhat redundant in my opinion. The results also don't reflect the grammar, as ultimately, only a transcript of the spoken text is obtained - particularly in Chrome.
Based on my observations (mid-2020), it seems that the current necessity for incorporating this structure is relatively low. While it may hold potential for future applications, the current status of speech recognition, as indicated by the Can I use... table, does not seem to be fully integrated yet, casting doubt on whether this will be the definitive approach to speech-related functionalities.