Is there a way to customise bookmark icons in Chrome?

I don't think there's a built in way of customizing the favicon, but you can use a chrome extension called Bookmark Favicon Changer which will let you modify it.

Q: Is there a way to customise bookmark icons in Chrome?

A: Not easily.

Bookmark icons are stored in an SQLite database in ...
%LocalAppData%\Google\Chrome\User Data\Default\Favicons.

The icon images are stored (in png format) in the "image_data" field in "favicon_bitmaps" table. This table is indexed on the "image_id" field (numeric). The "image_id" value can be found in the "icon_mapping" table via the indexed "page_url" field.

Additional notes:
1. sometimes there's more than one image per "image_id" (ie when there are different sized images - eg 16x16px, 32x32px)
2. the Favicons file can't be updated (by external applications) while Chrome is running.

There isn't currently (November 2019) a way to do this natively, although this functionality is a Chrome feature-request that I would recommend up-voting.

Your options at present are (from easiest to difficult):

• Use an emoji in the bookmark name
• Install a Chrome extension, e.g. Bookmark Favicon Changer or Favicon Changer
• Create a custom HTML redirect on your PC or server as per this article