<p>How to customize the color of the symbol inserted in the "glasses mode" in Emacs?
I have not found the option for this. Can anyone suggest anything?</p>

There's no option to set the face for the inserted separator (and from a brief study of the docs for emacs overlays, I don't think it's simple to add).
You can customize the face used for the capital letters that glasses-mode splits on; it's called
```
glasses-face
```
.