Prints in Black and White only
I have tried Chrome and Opera but cannot seem to get PrintCode to print in colour. It prints perfectly but only in Black & White so we lose all the syntax highlighting.

Same problem here, except that I tried with Safari. Does this extension actually do any syntax highlighting? Or can it pick the highlighting from other language extensions (e.g., Rust, ~~Haskell~~)?
Correction
Haskell source files are printed OK - with the appropriate syntax highlighting. Rust source files are not syntax-highlighted.
Help syntax-highlighting for Rust files would be appreciated!
Trying to do so for solidity as well if any tips
I have the same question for Chrome to print fortran code.
PrintCode uses CodeMirror to render files for printing. PrintCode's settings for particular file types are determined here: src/content.js, lines 10-26.
Not all the possible file types get any special coloring, so you may feel free to enhance the plugin within the limits of CodeMirror.
It seems @nobuhito is nowadays busy with some other projects along with vscode evolving fast, so we may need to take actions if PrintCode plugin is still deemed useful and worth of continued development. The last release of PrintCode was made in 2018, and in 2019 came for example this new plugin (haven't tried it yet): github.com/PeterWone/vsc-print.
Same for JavaScript files, it gives the colour coding for .js extensions but not for .jsxfiles and even it's not consistent behaviour across, have checked in other VS Code Project it's not working even for .js files.