Unable to get truecolor option to work

[[ re-sending this as I wasn't on the list when I sent it initially; ]]
[[ sorry if it ends up being duplicated                              ]]

I've been trying to enable truecolor support in my screen session after 
stumbling upon commit 9668a1ab00fea2096555a33cb962cc865b86e74a[1]. 
However, `:truecolor on` gives `unknown command 'truecolor'`, and

  truecolor on

in my ~/.screenrc doesn't seem to make any difference. Am I missing 
something obvious here? I've set my term to alacritty-256color (which I 
have a terminfo file for), and am testing with the command

  printf "\x1b[38;2;255;100;0mTRUECOLOR\x1b[0m\n"

which should show `TRUECOLOR` in an orange font. This works fine 
directly in my terminal, but not through screen. My screenrc is 
otherwise empty (that is, beyond `truecolor on` and `term 
alacritty-256color`).

Any advice would be useful!
Cheers,
Jon

 [1]: https://git.savannah.gnu.org/cgit/screen.git/commit/?id=9668a1ab00fea2096555a33cb962cc865b86e74a