I'm unable to figure out what the correct incarnation to specify the selection colour. Cursor colour I can manage but the selection colour remains a resolute light purple colour.

I've searched around on the xterm.js project too but can't find anything there either. This must be something thats been solved.
Add a selectionColor: <color> in your config :)
There is no documentation yet. 馃槥
I encourage you to backup your config file, remove it and launch Hyper. A new default config file will be created with all new settings.
Thats the one :)
Something I'm confused about is the versioning of Hyper. Around the web I see versions referred to as 2.1 and 2.0.4 but on the project there is just the 2.0.0-canary versions.
Thanks for the help.
We have changed our versionning and introduced -canary suffix.
All 2.0.0-canary.X are newer than 2.0.X and 2.X.X previously distributed on canary channel.
Sorry for this confusion
As @chabou mentioned, the solution is to add selectionColor: 'rgba(R_Intensity, G_Intensity, B_Intensity, Opacity)' to the config section in your hyper.js file (often found at ~/.hyper.js)
Most helpful comment
Add a
selectionColor: <color>in your config :)There is no documentation yet. 馃槥
I encourage you to backup your config file, remove it and launch Hyper. A new default config file will be created with all new settings.