How to print imessage chat

There may be third-party solutions, but you can do it from Messages using the usual print command.

Identify the conversation you wish to print, then use the option in the Menu bar: File > Print (or command & P). Then choose the option to save as pdf.

I notice the colour of the texts was lost in this translation to pdf (green or blue) but the text remains in bubbles and much the same as in the app.

Note that this includes text only a little way back. If you scroll backwards in time within the conversation to a point of your choosing then this too seems to be captured into the pdf.